I have been following this thread for a while, since the M812 is a Cummins. A local trucker/engine builder here, built a hypo 855 Cummins a few years ago with the help and advice of the guys in PA. They figured that they were getting nearly 1000 hp out of a stock block engine with a compound turbo, special fuel advance and and special pump. When I questioned him about RPM and head work, the only thing he said was that pistons are the key to high HP. They were using special factory pistons made specifically for the higher temps and pressures of the compound turbos. RPM limits were about stock @ 2100-2200. Heads and valve changes were minimal. He ran this engine in an older Pete for a few years, but it was hard on u-joints and rearends. Claimed it would spin the tires loaded @100,000# going up hill in the rain as the boost came up!
I guess my short point to a long story is that it seems that the pistons are more or less the weak link in these engines as long as you don't raise the RPMs.