I had a pre-facelift Cooper before the current S and I absolutely loved it
It was fine handling and had enough power to make it a lot of fun
The handling was so good it often felt like it could use a bit more
Unfortunately, with the Midland Gears gearbox on the pre-facelift Cooper, I wouldn't look for much more power; with 115 bhp it's already performing fairly close to its power limit. Another 10-15 bhp from a combination of a chip and induction kit is about as far as I'd recommend.
If it's power you are looking for, It's worth checking what it would cost to trade a still standard Cooper for an S before you start spending too much money on the Cooper.