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.
I do not have the specifications for making a "speed bolt", but I believe it called for the removal of as much as five ounces of material from the bolt body, leaving the bearing surfaces unmolested. Bronze bolts are ideal for this, as they are far easier to machine; unfortunately they have become fairly rare and expensive. The standard STEN bolt is extremely hard, so it will take some quality carbide cutters to machine.
John Andrewski could certainly do it, but I know he is very backlogged with work right now. Andy Blaschic I am unfamiliar with.
First thing I would try is a nylon spacer in the back of the receiver to restrict the travel of the bolt to the rear. The shorter the distance it travels, the faster the cyclic rate is. Nylon (or some other plastic) is cheap and easy to work with. The final design might be rubber os some material with more shock absorption.