I assume that Mass True Strike is identical to the 1st level version but effects 1 creature/level?
If so, I don't think that mass true strike would be much of a balance problem. It would serve to enable a few smackdown techniques (making the mounted paladin spirited charge smite smackdown somewhat easier) but most of those are corner cases and are functional enough without mass true strike.
Could it be advantageous. Yes. So can Mass Haste. Is Mass True Strike as good as Mass Haste? Is it as good as Disintegrate? Finger of Death? Summoning a Celestial Dire Bear (summon monster VI)? It's a 6th level spell, after all it darn well ought to be advantageous.
My estimate though is that far from being unbalanced, Mass True Strike is probably pretty weak for a 6th level spell. The only case in which I can see it being really powerful is if you are standing in the middle of a group of low level archer peons with one GMWed arrow each. That's not a very common situation. And, even then, it's not going to be really impressive compared to a circle of death, chain lightning, or a sculpted cloudkill.
Assuming my spell statting above, I'd probably peg Mass True Strike as a very strong 4th level spell or a weak 5th level one. For a 6th level spell, that's positively anemic.