No transformation is not a more powerful effect. Getting small boosts to stats and BAB are generally not worth giving up spells. A member of the warrior class is a significant reduction in party power and that's basically what the transformed mage is turning into.
Further the safe precedent for an ability score is +1 to a stat. While in some cases that can result in a +1 to hit, the actual effect is roughly +.5 to hit if you use it on your strength stat.
+1 to BAB is different that +1 to hit because a +1 to BAB can potentially result in more iterative attacks and/or give access to better feats.
I'd probably limit maximum safe effect to a +1 insight bonus to hit rather than a +1 inherent to BAB.
I see where your coming from, but I'm not so sure. The role that this (wish for full BAB) will be helpful is martial attacker. Transformation is more powerful in that regard. Not that it matters much, this isn't key to my point.
You are correct about +1stat, or +.5hit. For some reason I was thinking +1hit.
Getting additional iterative attacks or better feats is just more reason why this Wish would be too much.