I asked the same question a few weeks ago- I have around 11:1 compression and I have been running a mix of 93 octane pump gas and 110 race gas at about 2:1 ratio.
the engine seems to like it fine. I may could get by with a little less race gas, but I like to be safe.
VERY VERY good info especially if you are buying gas fro marinas with the alcohol content in the fuel now days it absorbs water VERY fast i seen a boat come in that got fuel lrom a marina at a local lake he has 3/4 tank of water and 1/4 tank of fuel
also the carb has a lot to do with the type of fuel and compression you can run if there is a place neer you to get E-85 i would probly go with a carb set up for it people are making big numbers with the e-85 and higher ethanol fuel also what type of booster's are in your carb will help determin that of corse an annular boost carb is better at atomising the fuel therfore making better use of the fuel there are guys around here running ump sportsman cars (360 inch limit any non roller cam spec heads 2bbl and gasolene) that are running 13to1 on 93 using an aerosol carb just my .02