You wont be disappointed with the S2, it is a snappy phone even on the awful Samsung firmware, but yes you will want to root and load a custom rom to give it that edge. I don't overclock (perhaps the rom does?) so I imagine that you can get even better performance of you choose to overclock. On DevNull my quadrant score is 4300 or 4400 (can't quite recall at this time).

However, at this stage the S2 is quite old so you might be better waiting for the Nexus Prime to see what standards it will set.