Radagast's "mastery" just might refer to knowledge of shapes and hues, or even some form of command over them, rather than any specific shape-changing ability.
I'm in total agreement with all of the above otherwise, but would like to raise one question.
After Gandalf's return, we start seeing visible displays of power. He breaks Sarumans staff, which I don't think even Saruman would have been able to beforehand (otherwise it would seem that he would have). He repels Ringwraiths. He takes a very active role in the command of Gondor's defence.
So.... was he made stronger, or was it just the case that the prohibition was relaxed (even a little) for him? And relaxed by whom? Almost certainly not Manwe, I don't think he would have had the authority to recall one of his own order from what was a very real "death". Did the Head Honcho Eru step in?
Thoughts & comments?