A slight change in fuel (5 or 10%) doesn't make much if any difference in performance. Experiment with props sizes, and you can usually improve flight perfomance of any airplane/engine. Usually engine HP numbers are given at RPM's that you do not reach with the recommend props. I have found that often you do better by dropping the pitch of the prop by 1 inch.

Irvine engines are great running engines. I ran a .36 for over 160 hours and pretty much wore it out. I like it enough for that size of engine that I bought 4 more. Be aware of the "block size". For instance the .25 and .36 were the same size and weight. Now they have punched the .36 out to a .39 displacement. Their next block size is 40 up to a .53 at the same basic weight. I would recommend alway buying the largest displacement in a block size for best performance.