Funnily enough, at high Accuracy they're actually not very inaccurate at all. The penalty for autofire and for dual-wielding two-handed weapons both only apply to the inaccuracy term from agent inaccuracy, which becomes negligible past Accuracy of 90 or so - at that point, all that matters is the inaccuracy of the weapon itself, and Devastators are more accurate than anything else except the Laser Sniper Gun.
Toxin C is a pain to get, yes. However, my table says that even B is 3-4x the firepower of the Devastator (and that's not including the fact that it ignores shields). C is more like 5-6x.
The Toxigun is horrifyingly lethal.