I think stopping power is REQUIRED for most SMGs. Obviously, the UMP is the exception since that one works better with another perk 2, since it kills quick already.
SMGs like the P90 and Mini-Uzi however, need the extra power from Stopping Power.
For example you would need 4 bullets if you use Silencer and don't have SP, which really is alot. Of couse, with the P90 that's less of a problem since it has a big magazine.
If you aren't using SP on the P90, I'd recommend the standard 'ninja' layout with Scavenger, Cold-Blooded and Ninja. I had a P90 steath class like that on COD4 and it worked great.