How much HP difference with compression

About 10 to 15 % assuming everything else is exactly equal. It depends a lot on the cam too. If the cam is in the 230+ degree @ .050 range it'll run like crap with 8.5 but run great at 10.5 with an increase in power well over 15%. If it's its a short duration cam (under 212 degrees), it may ping itself to death with no power increase.

This is a very difficult question to answer without some details or what it really is you're trying to do. Rule of thumb is 7% for every full point of compression.