Iron and oak is advertising different numbers for the fast cycle 20ton than you are writing. According to there ad's the fast 20 has a 4.5x2x24 cylinder and not the 4x1.75x24in cyl you suggested. To get 20tons they probably have the pressure set at around 2500psi. Also the 8sec cycltime advertised should be about 11.2sec, give or take a few billion years. Anyways, if you copy their design and use the 4in cyl with smaller rod dia, you would have the approx 8 sec cycle time (closer to 9 sec) you are trying to match. Going to a cyl with a 2in rod will only speed up the cycle time by .2 sec, not worth the effort or extra cost of the bigger rod. To keep heat down to a minimum without a lot of extra expense, I would look for the cyl with the 3/4 ports as well as try to find a splitter control valve rated for 25gpm. the bigger ports in the cyl and valve will reduce the velocity of the oil flowing back to the tank on return stroke, which in turn will reduce heat. I would also try to build or find a hydraulic tank in the 15gpm range. Set hyd pressure to around 3000psi which would give you about 19tons of splitting force. You will need a 8hp min engine to pull the pump. To increase splitting speed, I suggest using a 4way wedge, you can make it removable or height adjustable for those few times you run into a round you cant split. I can say for certain the 4in cyl with a 4 way wedge is going to hard to stall. If you need cycle time faster than 8 sec, you can always go with the 22gpm twostage pump and a little bigger engine, but if you can keep a 8sec cycle time fed wood, your a better man than most of us.
