I have discussed this and some of the more experienced members here expressed caution when going too far over the factory specs like the 12.7 gpm. I have a 11gpm pump on my build but I have yet to test it. I have seen builds in the range of 9 and 10 gpm and it makes the tractors quite a bit faster and overall wouldn't change a ton of things other than the possibility of hearing some fluid whine through the relief on the TCV, possible requirement for more cooling capacity, and also the possibility of the pressure not matching stock specs which I believe are 2300psi at the pump? If it were me and I planned on not doing anything beyond what is factory-intended with the tractor, I would keep close to factory so the 8.5gpm.
Odds are at this stage you'll have to make some sort of adaptor and get special lovejoy connectors to adapt to the Honda motor, it is what it is. As for the hp, 20hp is plenty for either of those pumps operating at 2300psi. I use the below link for checking hp requirements:
Calculate the Hydraulic (fluid power) horsepower.
www.metaris.com
