I don't post often, but we've entered the realm of military history.
The advantage of a bow is not that it kills with a single hit. The advantage of a bow is that you can fire it at distance. Conservative estimates on medieval longbows give them 200 yards (600ft) of range, while reconstructions of bowstaves recovered from the sunken Mary Rose have reached over 300 yards (900ft).
Given that a 'tile' in Avernum isn't probably more than a squad meter, you see the problem. The advantage of the longbow is pretty simple to calculate. 200 yards is just over 11% of a mile, the world record is 21 seconds and change. Expect a man carrying his own equipment and armor to be slower, closer to 45 seconds. Expect men (plural) in formation, to be slower still.
Add in mud (which is not unfair, contrary to popular belief, muddy battlefields are the norm in Europe, not the exception), and you have more than a minute to toss out fire. That does, quite literally, mean almost a dozen shots.
If you wanted to make bows realistic in Avernum, you'd either want to increase their range so that it takes 10 turns to cross, or to speed up bowfire, so that 10 shots could happen in the time it took a normal monster to close the 10 or so tiles of range you have. Either one would be a game-breaker.
As to lethality. Being struck with an arrow may not kill you, but it will absolutely render you 'out of the fight' instantly. It's not the force of the arrow but the shock (in the medical sense) of being struck.
Now, from an actual gameplay perspective...I'd suggest a slight upping-of-the-damage, to bring it inline with the damage from swords, but still less of pole-weapons.