Well performance is much better, this is really a very broad question, go to Apple's website of the new 2011 MBPs and compare the tech specs with the old one.
I wouldn't suspect the new models would have overheating issues, after all, any computer is vulnerable to overheating if used irregularly. Not sure what you mean by flickering.
My experience re: heat with the new 2011 is that at one time it was pretty hot, but with the 10.6.7 update heat reduced drastically. It isn't really hot at all.