That's fair. I have to admit starting at +3 came nearly directly from Frank's original comments in this thread. IMO he made a lot of sense when he pointed out that 1 level of any arcane spellcasting class (Bard, Sorc or Wiz) carries with it a +0 BAB for that level. With a 3/4 BAB progression here, that first level of AA means the AA is effectively down 2 BAB already compared to a full BAB class.
I was really trying to push the "bow specialization" angle throughout the build, and by starting with a +3 enhancement on the mehta an AA is better than almost anyone else with a bow. I say almost because a straight 8th level Fighter with Weapon Focus and Greater Weapon Focus will have the same to hit bonus using a mundane bow, while with a +2 bow (not unreasonable at 8th level, naturally or using GMW) the Fighter still outshines AA by a couple attack bonus points. The Fighter will likely have Weapon Specialization as well, and with a mundane bow that eats up 2 of the 3 enhancement damage points the AA has going for her. Using that +2 bow the Fighter surpasses the AA's damage by a point.
So, to sum up:
8th level Fighter with WF, GWF & WS, using a +2 bow has an attack bonus of +12, with a damage bonus of +4.
A 1st level AA (of 8th character level) gets a +10 (+1 from the WF prereq, and +3 from the mehta), with a damage bonus of +3 (from the mehta).
Add to that the Fighter will be nearly as good with any other weapon, while the AA without her mehta drops her BAB to +6 (+7 if it's another bow, because of WF) and does no additional damage.
The +3 weapon as early as 8th level is significant, but not overpowering. Doing it this way serves two purposes: the AA is very specialized with her mehta, and therefore dependent on it; and the mehta gains some much needed protection against a common tactic used against archers -- sundering.
If the AA could pick up any bow, at any time, and freely switch between them, I'd lower the the mehta at 1st level to +2. But tying all the powers to a single chosen bow, with all its ads & disads like the Mageblade, means a +3 start is doable and still balanced IMO.
Thanks for comments, Nifft. I like your build a lot as well. It's based on a different perception of what an AA is, I think, but the execution is good with some great ideas!