If it is a second generation Cooper (R56) then a very easy upgrade is to switch from 280mm Cooper discs to 294mm Cooper S discs. These use exactly the same brake calliper and so you don't even need to touch the hydraulics. It needs:
- Cooper S discs;
- Cooper S pads;
- calliper carriers/brackets for a Cooper S (about £70 each from a dealer);
- possibly, brake wear sensor.
Fitting is no more work than swapping pads and discs.
This improves the braking from mediocre to decent. Clearly it doesn;t look very different, so if it's actually looks, not braking, you're after, then this isn't for you.
If it is a first generation Cooper, I can't comment.