My MK-V is noticeably faster than my MK-IV (still not real fast, but not slow like the MK-IV). It has a lighter bolt and a single spring (obviously to compensate for the pressure that's reduced by the "bleed holes" in the bbl). It also included a rubber buffer, but IIRC it didn't run with the buffer installed. But adding an extra buffer to my MAC M-10 (.45) sped it up, not that it needed it. Maybe try a rubber buffer in the MK-IV? They don't hit a buffer like the MACs, but it would increase the spring pressure a little? Plus it's cheap and removable.
Otherwise, I'd just talk to John Andrewski or Andy Blaschic. They might have the info, and either is probably able to lighten a bolt. JMHO FWIW.