My MPX' mod list
Hiperfire eclipse
TTI buffer tube conversion with Geissele tube
Hogue Overmold stock
Magpul K2 grip
Radian Raptor charging handle
optics...
Otherwise, my MPX' are internally stock. Keep the gas system clean. A clean gun is a reliable gun.... I've found that once you modify the internals, issues start happening. To the original question, I'd agree with Tony that muzzle devices basically don't do anything other than deflect noise back. The MPX is an amazing shooting PCC and I'd think it would be tough to improve with mods. I'd say the better thing to try and achieve is 100% reliability. Ammo being used will help or ruin that percentage.