[Johnny .45]

Okay, I think I mostly follow you here..."mass distribution", i.e. how far apart the weight is along the wingspan. The more weight you have out in the wings, it will effect roll rate...we had this discussion on a different site regarding the P-38. The way I read it was that the P-38 was a quick enough roller, but it's RESPONSE to the control input was slow...with those heavy engines out on the wings, it took a moment for it to "accelerate" into it's full roll rate. Like a heavy car can have the same top speed as a lighter car, but for the same power, the lighter one will be far quicker "off the line".
Cannon would have a similar effect...between a .303 and a 20mm-armed Hurricane: the wingspan is the same, the ailerons are the same, so the force the ailerons produce is the same, only it's trying to move a much heavier cannon. Friction isn't an issue, so the only effect is that it builds up roll-speed slower.
But part of what I was saying is if a pilot has to use slight OPPOSITE deflection to STOP the roll (the momentum will try to keep the roll going), it will be all the more sluggish on STOPPING the roll. Like the lighter car can out-accelerate the heavy car, it can also stop quicker too. Momentum and inertia involve exactly equal forces...there IS no difference, in terms of physics. An object in space doesn't "know" it's moving; in fact, I think it's aeronautical-engineering lingo to call momentum "reverse acceleration".
But I don't see how cannons could INCREASE the maximum roll rate. The maximum rate would be the same, it would just take longer to build up to that rate (and longer to stop). If heavy cannons increased any performance parameter, it'd be dive-rate! A little extra weight to increase the terminal velocity...although I'm not sure whether in that situation more weight would make a lot of difference, or if it's more about the power of the engine.
I'm curious about that now, too! If a plane can dive at 500mph at full power, how much slower would it be in a glide? The Thunderbolts impressive diving ability makes me suspect that the weight of the plane is important, and I'm not even sure if a prop on full power at 500mph is helping to propell the plane, or if it's just contributing to drag at that point!
Anyway, I guess we can chalk "roll-response rate" up as one potential advantage of a fuselage-gunned fighter over a wing-gunned fighter. All other things being equal (using the same plane, etc.), a Bf 109G with wing guns fitted would be slower into a roll than a Bf 109G withOUT wing guns. Of course, we (mostly) all have heard of how the Bf 109 "Kanoneboote" was disadvantaged by it's wing guns, so now we know why. Although, of course the projecting underwing gondolas would create a little drag, too.