I agree with a rate of fire increase as far as general buffs go, it'll be a useful choice on gattling, light carronade and maybe others with that. However as far as breadth of use goes it seems like that would buff mostly the guns its already almost effective on.
The opposite could be interesting - an even more severe nerf to rate of fire and clip size and a flat increase to stack size and ignition chance, it would buff it in general and nerf it on the rapid fire guns its currently almost good on.
@Exterminator - Whats so complicated about it. Bigger shots cause bigger fire stacks (it makes sense for 1 merc shot to carry more incendiaries than 1 gattling shot). As far as implementation its just an extension of how they implement fire weapons allready. Ill admit that its a lot of numbers to balance but inverse of hit rate (average rate of fire over entire clip cycle X Blast radius) would give a good starting point from which to nerf the effect on guns that become too powerful with it.