I am working on an upgrade of my northern splitter now. I am changing the 5hp engine to an 8 hp honda and the 11gpm pump to a 16gpm. I have concluded, once finished, the upgrade will be about $500. That is buying a used honda engine at a big discount. You need at least a 8hp engine to run a 16 gpm pump.
I still have to investigate the fluid temp issue. The cylinder is staying the same, and based on some free calculation "software" my cycle time will go from 14 second to just under 10 seconds.
There is a supersplit on CL locally for $600. It has prcessed over 400 cord of wood though. I like the power of hydraulics and am not in that much of a hurry to use a SS.