James Whiting did this upgrade on my car last year and I'm very happy with it. I've done quite a few trackdays this year and given them a pretty hard time with no problems with the standard discs and no fade. The big advantage over the standard system isn't really absolute stopping power, it's about how much more feel you get. The standard brakes will stop you very quickly if you stand on the pedal hard enough, you just don't know if you'll lock a wheel. The alcon setup means you can use the brakes very, very hard and be confident that you won't lock up.
A worthy upgrade and very good for braking confidence, especially on track.
Tim