Roll rate's not the issue. You have so much that even with heavy guns packages, you're still outrolling most planes in the game.
Oh, and the MG151/20 outboard package almost weighs as much as the Mk108 package does.
The problem's not roll, but that sluggish feeling where you can't pull a higher angle of attack than the A-5 can, or the D-9.
That's just part of the plane. You either take the guns, take the weight, or leave the guns and the weight. Many times the extra weight is worth the firepower advantage. But it's like taking the gondolas on a 109: there are drawbacks.