@Grummz you know I luv you.
the problem with rng in terms of weapon improvements, of any kind, is the big question:
If x is weapon improvement factor, and one player has 2x, and another has 5x, is player one doing more damage than player another assuming all other variables are equal?
if the answer is yes (and from the looks of it that is the intention) another question arises: will content become more challenging as you progress as a result of x?
This is where problems arise: If you do factor x in difficulty, players will be expected to get 5x weapons for group content or else die a lot or not even be welcome in most groups. If not, you have players with the 5x mod complaining about everything being easy and the others complaining group content requires players with the 5x mod for successful completion. In either case, you risk have issues regarding, dare I say it, P2W.
In summation, tread carefully down this path, for the hanging fruit of chance improvements in igs and the like may well be poison to your reputation and much of your community.