I might be crashing the party a little here, but isn't the available ammunition for each weapon a key balancing point for numerous weapons? The primary goal being that players need to be conservative and stick to their roles to get the best yield out of the weapons, rather than just spamming shots at anything that moves.
The key example being a Support Specialist since he would not even blink at an extra 3 weight blocks, and he'd recieve 30% extra on both his weapons for his natural perk bonus, and then 10% or whatever on top of that. Plus it would be an absolute given for Firebugs, since 95% of my fellow Firebugs cannot seem to resist holding their fire button for at least a second a go and whine when they run out of ammo.
I dunno, it's an idea that has promise if the ammo balance was altered accordingly, but as a verment believer that players should be conservative with their ammo, and deliberately target only those specimens their perk is intended to tackle, it sounds to me kind of akin to weapon stacking. On paper I just can't shake the feeling it encourages a more spammy style of play...