Gas Question, What effect?

Actually you'd make less HP with high octane gas. The higher octane slows down the burn to stop detonation. It also slows down the burn during combustion, hence, less horsepower. If you aren't detonating with 91 octane you dont need anything higher. You might try 89. If it doesn't detonate you might make a couple more hp and save a few bucks to boot.