I'm not surprised that going from 0% to 10% had a huge change. That's a huge jump in nitro, and the changes you noticed are exactly what you should expect. Going from 10% to 15% will show a much smaller change, and unless you're looking for every last RPM, going over 15% is a waste for most guys.

You mentioned changing plugs with nitro. More importantly, engines set up to run on 0% fuel usually have higher compression than engines set up for more nitro. So, if you want to run 0% nitro, you can benifit by removing the head shim or going to a thinner shim. On the flip side, many European imports are set up for 0%, and to get them to run smoothly on American 15%, adding a head shim does wonders. As I understand it, nitro is much more expensive in Europe than in the USA.

Also, differnet brands of fuel do perform differently. Mostly the differences are in the oil % and the types of oils in the fuel.

Lower oil % means higher power, as there is more burnable stuff in there. But some engines like more oil and will actually pick up RPMs if you increase the oil to a certain point. Also, some oils are just better at lubrication than others, so a smaller % of some type of oil will work better than a larger % of another, and leave more burnable fuel, resulting in better performance all around. There's a guy in my club who runs custom fuel with only 10% oil, and he never burns up engines. But he knows his engines, and he doesn't set them lean. I, on the other hand, like the insurance of a bit more oil, but I don't run 20% either. I do like a dash of caster in my fuel though, so I run Morgan Omega, which is roughly 17% or 18% oil, mostly synthetic, but it does have some caster.

To top it off, air temperature and humidity can really affect engine runs as well. And some fuels will perform better than others in different weather. For example, on a high humidity day, you might want more or less oil than on a dry day.

The good news is that 99% of this only matter when you really are pushing engines. Even when I fly competitions, I still run good ole 15% sport fuel just to keep my life simple. I might give up a few hundred RPM, but that's ok for me.

Btw, there is lots of good fuel and engine performance info on the powermaster website. I seem to have lost the url, but they have a FAQ on there that was written by their former owner that is almost a "must read", even if you never buy powermaster fuel.