Personally I always felt the Headshot multiplyer was too high on the Xbow. IMO Sharpshooters should be the ones dealing a HUGE amount of damage to the Fleshie, but certainly not 1 shotting them.
I'm not expert on the code, but as far as I understand it, headshots deal damage to both the body and the head, who each have their own values. All damage done to the head is subtracted off the head health of the specimen and the regualr health.
Heads obviously have less health than the actual specimen, 6 man suicidal Fleshpounds have the highest at 3675 Hp. (Scrake is only 3418HP).
Now if I remember rightly the Xbow has a 6x headshot multiplyer (Someone batted around 7.8 somewhere, but I'm using 6 since it only amplifies the arguement if I'm wrong). Unperked the Xbow using this model deals 300 for a body shot and 1800 for a headshot. That is pretty damned high in any perks hands
Put that in a Sharpshooters hands and you get 1800 x 1.6 x 1.5 = 4320 damage in 1 headshot... which is what makes the Xbow so unbelievably overpowered.
Only class that even gets into the league I believe is Demo with 2400 damage on a pipebomb, and 1496 on a L.A.W. headshot. Even Support with his double Hunting Shottie gank is only 950 and that is assuming every pellet hits for a headshot... (think they have a 1.1 headshot multiplyer... which if not makes them even worse.)
At the very least Xbow should to be brought down to x5 headshot multiplyer which would give 3600 damage. Not enough to decap a 6man Sui Fleshie in 1 shot, but it robs it of over half its health, and 1 headshot with pretty much anything else will decap it. Of course it will still be enough to decap a Scrake, so seems about right to me.