Can't tune propane carbs(mixers) worth a crap. They do not run at ideal a/f ratios at full throttle. A propane injection system would be the way to go, but propane injectors are not cheap.

Propane has a lot of octane but produces less BTUs of energy than gasoline when burned. So you build to a higher compression ratio to use the extra octane, but produce no more power on the dyno because of the lower btu rating.

Some people are making great power on propane, but for most, the cost and headache of it all is just not worth the trouble. I calculated it out, and back when gas prices were high, it was not THAT much cheaper to run on propane than gas. You would have to be a delivery driver or do a ton of driving to make that investment back.