Some upgrades are worthwhile, like the current style H&K ejector, and the 80 degree locking piece on a suppressed "K" PDW.
If you really want to spend some money, the M.A.D. Max or R.C.M. upgrade bolts have a vastly superior extractor which uses a coil spring. The design of this extractor and its spring were "lifted" from a couple of H&K belt-fed LMG's so it is a proven, durable design. This eliminates the MP5 Achilles heel, the failure its OEM "bent wire" extractor spring.