What is the performance difference between a 18X8 prop and a 18x6-10 prop? In other words, why is such a prop as a 18x6-10 prop with pitch change as you go to the tip used?

It's a compromise. In theory you could otimize different portions of the blade at different speeds for a wider range of effective prop and plane speeds. In some set-ups it may give a better all round pull.

I've never had a set-up that worked better with the 6-10 than just a straight pitch with the right diameter, though.

CJ

You will get a bigger difference with the same size propeller from differnet manufactures.