Mastering an aircraft, and being ready to move up to a more nimble plane are two VERY different things. 

One can spend YEARS flying the same aircraft, and never really MASTER it.  I do feel, when you can safely, and consistantly perform basic rolls, loops, patterns, and grease darn near every landing it's time to move up to something a little hotter.  Years back, I knew it was time to move up when I was bored out of my mind flying my trainer.  I could fly that ting in my sleep, however I Still own my old Royal Air 40T that I learned to fly on...and to this day I don't think I have "mastered" it. 

LOL, just for giggles I'm gonna take it out at some point this week </p>