Generally, you don't install a dual fuel system for the performance. Obviously, the main reason to convert is to save money on fuel and this only makes sense if you drive enough and there is a big enough spread between the prices of the two fuels. Even with the slight loss in power, there is still plenty available for normal driving. I have a sample calculation you can do on my conversionspage to see it it makes sense for you.

You do regain some of the power of the engine by increasing the compression ratio and 10.5:1 is a realistic limit because of propane's 104 octane number. However, you are right about this not being realistic for a dual fuel engine because you would also have to run high octane gasoline. The performance loss actually comes from the gaseous fuel displacing some of the air required by the engine rather than from its lower energy content per gallon. Interestingly, propane has a higher energy content per lb but its lower density results in the lower energy content per gallon.

As far as performance goes, I know of one propane drag racer. Gas Injection Technologies in Australia have conversions(not available here) that outperform gasoline systems.