So, regarding GosJohn 17.
The first thing to keep in mind is that orthodox trinitarians actually do marshal verses 1 through 8 (and related verses throughout this chapter, this discourse, this Gospel, and the New Testament) as part of our overall case. As you noted, there is certainly a distinction of the persons being evidenced here; orthodox trinitarians positively affirm this over against the modalists. Again, the Father is declared by Jesus to be the only (or maybe the one) true God; orthodox trinitarians positively affirm this over against various Christian groups who would deny in various ways that the Father is the one true God.
The personal pre-existence of Christ is affirmed by the orthodox, of course–which is sometimes denied by various Christian minority groups, who would at most consider Christ to be some kind of plan or concept prior to his birth. Moreover, Jesus claims to have been sharing the glory with the Father before the kosmos came into existence. In a Jewish context, this would be a ridiculously elevated thing to say: the glory of God is the eternal shekinah, itself tantamount to the very presence of God. It is one thing for God to share the shekinah with derivative creatures within time–such as what God reportedly did in the tabernacle and the temple during OT times before the Diaspora, or such as what Jesus promises toward the end of John 17 (and elsewhere). It is quite another thing for a person to declare that he has been sharing the shekinah with the Father before the kosmos came into existence. That kind of declaration isn’t about recusing to a prior time-before-time, but a shared ontological existence transcendent to the totality of creation at all. (Admittedly, someone could claim this about an entity, or about themselves, and try to be meaning something less than sharing corporate existence at the level of God’s own self-existence. But insofar as Judaism is concerned, they would be making a pretty damned daring, or maybe incompetent, hyperbole by doing so.)
It should also be pointed out that Jesus (and/or maybe the Evangelist, who likes to insert commentary asides) claims that eonian life (zoe eonian) consists not only of knowing the Father, but also in knowing the Son. In other words, I am personally receiving life from God when I am in personal fellowship with God–which is straightforward enough–but then if Jesus isn’t God, it makes no sense for the reception of God’s own life to be dependent on my fellowship with Jesus. (And even less sense for Jesus to be claiming to be the Life!–as he does elsewhere at least twice in GosJohn.)
Jesus is at least being treated (by the author and/or by his own testimony) as co-source, with the Father, of ultimate life: the Prince or even the Author of Life, as St. Peter puts it in his famous first sermon (Acts 3:15). This makes sense if they are somehow both YHWH, the self-existent Who gives life to derivative creation. It doesn’t make sense if Jesus is only a super-angel (Arianism), much less a super-Moses (neo-Arianism).
So, in John 17, we have Jesus affirming, as a distinct person, that the Father is the only true God; and affirming that he himself was pre-existent with the Father sharing the Father’s glory; and affirming (or being affirmed by GosJohn author commentary) that zoe eonian comes through fellowship with himself as well as through fellowship with the Father; while emphasizing that the name of the Father (YHWH) be kept in highest honor: a name that he says the Father has also given to him, the Son.
This comports very well with other material in the Final Discourse which points in the direction of the Son sharing an ontologically primal Shema unity with the Father (as noted in various ways earlier in my comments for this thread), even though the Final Discourse chapters do tend to emphasize the personal distinction of the Son compared to the Father in at least a hierarchical subordination of the Son to the Father. Ditto GosJohn more broadly (as also illustrated in various ways in my previous comments for this thread.)
As a comparison in two different strands of epistolary material: In 1 Cor 8, St. Paul affirms the Shema (there is no God but One–keeping in mind that in Hebrew the word for One would be a compound unity); affirms that there are in fact lesser lords-and-gods than YHWH; absolutely distinguishes between God the Father and those lords-and-gods; absolutely distinguishes between Jesus Christ and those lords-and-gods; affirms that God the Father (the one God compared to those lesser lords-and-gods) is the creator and sustainer of all things; affirms that Jesus Christ (the one Lord compared to those lesser lords-and-gods) is the creator and sustainer of all things; and tacitly affirms (by shifting the application of the one-Lord-title, previously professed of YHWH, to Jesus) that Jesus is a person distinct from God the Father.
The Epistle of Jude treats Jesus Christ as a person distinct from the Father; and affirms him to be our only Lord and Master; while also (in close proximity to both claims) referring to the “Lord” Who saved Israel from Egypt. (Unless standard text-crit principles are actually correct here, in which case the text of Jude most likely originally read that “Jesus” saved Israel from Egypt!)
The basic themes are the same in each strand of canonical tradition, though expressed rather differently.
How important is it that Jesus claims the Father to be the only true God? It would be far more problematic for ortho-trin if Jesus was overtly denying to be the only true God! It would not be strictly a problem for ortho-trin, on the other hand, if Jesus testified that the Father was his only true God. (Which Jesus is not reported saying. That the Father is his God, yes. His only true God, no.)
If that one statement (“it is eonian life that they know Thee, the only true God”) was all we had to go on (not counting other material even in 17:1-8ff), then (by tautology) we would have nothing indicating Jesus claiming (and canonical authors claiming about Jesus) an identity tantamount to also being the one true God. But, to say the very least, we do have other material, including in 17:1-8ff. But also including Johannine material such as 1 John, where Jesus (certainly at 1:2 and arguably at 5:20) is referred to as “Eternal Life”–and thus also “the true God” per v.20! 1 John 5:11-12, relatedly, states that to have Jesus in one’s heart is to have life eonian. Again, the Father is called “the True” at least once in 5:20 (a long verse with a lot of interesting claims), and by grammatic context so is Jesus Christ the Son in the same verse. (Jesus is called “the True” again by the author of RevJohn 3:7.) Indeed, in John 14:6 (earlier in the Final Discourse), Jesus proclaims himself to be the Truth, as well as the Life–while distinguishing himself personally compared to the Father.
So the statement from Jesus in the opening verses of John 17 can be best understood as exemplifying (if less obviously than some other material) the Son, as a distinct Person compared to and subordinate to the Father, sharing the attributes and authority unique (per monotheism) to YHWH alone. And the opening verses fit in well enough with the same exegetical testimony found elsewhere in GosJohn, in Johannine materials, and in other strands of NT testimony (Pauline and “other”. 
) But particularly in Johannine material, insofar as some particular thematic thrusts are concerned.
