With the K, SBR'ing it so you can use the K grip is pretty much a necessity. The full size MP5s just shoot better to me though, and aren't that much bigger.
I wonder what rollers they have in those if you are getting that much action noise. My HK is stupid quiet with a can on it, and not much action noise.
I know nothing about mp5's, but how hard would it be to upgrade the action? New rollers and what not.