I'll just toss one more option out there:
A drum brake. I don't know what the weight penalty would be compared to a disc conversion, but they pull the same amount of cable as road levers. They stop well, and a new wheel much simpler solution than trying to get a disc fitted.
I used drums on my Swift and was extremely happy with them. They stopped fine (on par with a good set of v-brakes IMHO), weren't affected by weather, needed no maintenance, and I didn't have to worry about the rotor getting bashed while the bike was folded and in transit.
It doesn't really jive with the 'high prefomance' feel you are looking for, but that's probably a matter of aesthetics more than function..