Even a couple of hits with iron x sounds like a start. Surprised Ron didn't recommend that. Avoid harsh wheel cleaners.
I've always found that iron x is only any good for very lightly baked on stuff. Even the paste has limited results.
I know acidic cleaners are frowned upon and I very rarely use it. But for me it's a last resort and once it's used once there's no need to use it again if you keep on top of cleaning the wheels regularly.
James was saying that his wheels were really bad with proper hard baked on material so I doubt iron x will do the trick.
@Daniel linked me up to some genric acid based wheel cleaner that's obviouidy quite harsh but really does the trick on stubborn wheels.
One of our beemers at work had awful baked on s**t and I'd tried using iron x and iron x paste to get it off but it wasn't touching it. One hit of this acid wheel cleaner and the wheels looks brand new. You just have to work quickly with it.