I don't think bows are too broken at str +2. Several reasons.
1) under normal circumstances (BBB rules only) you can fire them at most once a turn.
2) Sure, many other weapons powered by strength use str/2 + X. But bows don't compete with these weapons. They compete with guns. (They are ranged weapons and are the same size as a largeish firearm).
3) They are at most mediocre for almost every character, it is only the characters that start to get into the str > 10 range that really make bows start to become more deadly than guns. Consider for a second what the implications of making their damage code str/2 +2 would mean for the worlds currently strongest man. Lets assume a strength of 7 (exceptional attribute). He therefore can do 6P -0 once a round with a bow. Or he could pick up a heavy cross bow and do 7p -1 twice a round. So therefore this would make STR powered bows all but useless to every human and elf. (even at str 9 they do the same damage as said crossbow, but have half the rate of fire) so maxing out your str therefore provides no special bonus to ranged combat (It should, str=9 is SUPERHUMAN! lest we forget.) Sure 11P -0 does seem a bit extreme, but you can only fire it once a round.
a) Direct Comparison to guns: There are only a few guns that have similar rates of fire the the bow in the BBB. They are the warhawk, the PJSS elephant gun, and the Panther assault cannon. Of these the PJSS is a good approximation for the bow due to its similar use (when compared to str min 10+ bows) as both of there legitimate purposes are in big game hunting. For comparable damage, one would need to achieve a str score of 7, which of course is not too dificult for trolls, and only a moderate investment for orks, but a dramatic investment for humans or elves. One difference is that the PJSS costs around 10 times as much. However, Suzie the paraplegic 80 lb. 10 year old girl can fire a PJSS (from a prone possition, but of course it has the range to allow that luxury). The investment in increasing one's strength to levels significantly beyond the damage achievable with the PJSS is significantly more than the 1.2 BP that a PJSS costs. Not to mention that normal ammunition for a bow costs as much as Explosive rounds.
b) The PJSS in not the best gun out there, bows also have to compete with sniper rifles (which can be loaded with APDS), that have twice the volume of fire of the bow and significantly longer range. (although they are of admittedly distinctly different legality classes.)
4) The 18P bow is a special case that might as well be magic for the difficulty of getting 5 points of strength enhancement to stack past ones natural maximum. (I can't see how to do this without magic, unless you actually do find a way to raise the natural maximum)
a) Realistically 13 is pretty much I can see sane (and that is kind of questionable) PC's getting in strength which means 1 attack per round at 15p -0 (+1 for explosive arrows or whatever they added in arsenal). How is that so much worse than 2 8p -4 attacks in the same amount of time? (Walther MA 2100 w/ Ex-Ex) I mean, is it really worth getting ones strength up to 13 just so you can make one attack at twice the value of the attacks you could be making with STR = 1 twice as often that also have much higher (lower) AP ratings. One could also compare the bow attack to a short burst from an alpha followed up by a grenade (from its launcher). Which has the distinct advantage of being area of effect.
Overall point: Bows are not unbalanced as they are no better than other options.
So, essentially, I think that the fact that bows are one of the only really useful applications of high STR, (which is really easy to obtain with a troll compared to achieving such high scores in most any other attribute) just means they attract more attention to the uses of trolls' overcompensated str score than other things. But that isn't to say that there aren't more broken things out there. For example Subduing Combat + Crit. Strike 6 beats a bow by at least 3 points of damage every time. Plus your opponent is immobilized. (Subduing Combat damage is calculated based on straight STR, just like bows are.)
If you really want to talk broken (but not abused, fortunately) for the investment of just under 500k

you can get an Ares Dragon(slayer) which has a damage code of 66P -half impact, and are reusable. (with good piloting skills you will succeed on the crash tests, thereby avoiding actually crashing, they are only threshold 2, before the VR threshold reduction) More probably: Cesna's can be purchased at chargen (~160k

) and can do 54p -half impact.