When changing compression from 8.75 to 9.5 of 9.75 what should a person see in the difference of HP? Base motor is 500EFI. Is there a rule of thumb as to how much HP gain per .25 increments?

I just did that too my motors over the winter and took them to 9.75 comp. , everyone said I should have with the other mods done, cam, porting, carb.....should be around 580-590hp.

usually a longer duration cam accompanies a hike in compression ratio.
Some combinations could actually lose power with a hike, if you were running close tooptimal cylinder pressures already.

Hot Rodding did a article awhile back that started at 8to1 up to 12to1 averaged about 4-5%increse per increase. Of couse
cam, heads and other factions will come in to play.