I'd say you probably just need to re-tune the engine to accomodate the new fuel. Otherwise it could be that the new fuel you are running has a higher oil content which will bog down the engine more.
It does sound like its still a bit rich. Rich is good but don;t run it too rich as this is just as bad as too lean.

You could go up to 25 or 30% which you will see a slight increase in power but at the expense of the engine's life in most cases. Its a personal choice really. I personnally like running 25% and thats it. Also you should be able to use the one fuel for all your vehicles of course thats if they are all running the same % nitro that is.