Since Infinity Ward doesn't really know how to do it, how about some of the wiki community's ideas for weapon balance in MW2? First, state the weapon/whatever, specific and ideally realistic reasons why you think this weapon/whatchadoodle may need a nerf.
Personally, I believe that it's fairly obvious the UMP, relative to the other SMGs, is way overpowered. It has a 32 round magazine, larger than any but the P90, and kills in 3 shots without stopping power at any range, which is as much as any other SMG can manage with stopping power. It has less recoil than the P90, MP5K, and Vector, is far more ammo efficient than any of them, and can beat any of them with Stopping Power at any range when it has Stopping Power, and manages to beat them at close range without SP most of the time.
Thus, I think a good idea for a simple nerf would be to reduce its damage to 35-25. This would allow it to retain its long-range advantage over other SMGs, while making it less effective in CQC than any of them, mostly by keeping it from increasing its close range TTK with Stopping Power. This would also make it so it's not a one-shot kill from any range in hardcore, making other SMGs a more competitive option.