RUNWME:
This combination seems ok for top speed, but it turns WAY too many RPMS for my comfort level.
In addition it doesnt get on plane very well(blowing out prop).
I think you have a good all around prop. You can hit 7K on gas and go to 8K on alcohol. That is right in the range you should be in. The 2.5L Hi Perf. motors are designed to turn 7,500 RPM. Why do you want to spin less than that on alcohol and only spin about 6K on gas. If you go to a 30" prop, or 32" prop, your acceleration is going to be for sh*t and the boat will be extremely sluggish out of the hole. Sure it will bring your RPM's down when running alcohol, but it is going to be a slug on gas.
You will not find a good all purpose prop for your rig. There is too much of a discrepancy in HP when you run gas vs. alcohol, thus you need multiple props! I would spin a round ear 26" chopper on gas. You could try either a 3 or 4 blade. This will give you good acceleration, better out of the hole, and decent top end. With a 3 blade 26", look for RPM's in the 7,600 range. With a 4 blade 26" look for RPM's slightly less than that.
Keep the 28" and throw it back on the rig when you run alcohol.