Get rid of the underdrive crank pulley and put one of these on.
ATI - Super DampersŪ for Mini Cooper Engines
You need dampening on the crank pulley. I'd also inspect the belt tensioner and put a safety stop on it.
Also, if you're going to track it, change the suspension bushings to polyurethane and reinforce the upper strut mounts.
They respond very nicely when you upgrade the intercooler, do a little smoothing out of the intakes and custom software.
PM me if you have any questions. I know these cars too well. I work on at least 2 a week.