Just back from Road Atlanta with Peachtree BMWCCA. Like any good safety product that works, once you're on the track you don't notice the harnesses. They work great and hold you in place without sliding. I still wish I could make the lap part tighter. I have both sides of the lap belt as tight as they go, there is just no adjustment left and you can make it tighter by moving the seat up or forward, but obviously you need to maintain a decent distance from the steering wheel and pedals and the other problem is it only tightens one the side that's fixed (the inside belt buckle).
Anyway, overall positive review and no regrets. I'm not sure that an M3 specific application is going to be any better as there is a limitation in how little belt/webbing can be used between the two buckles, both male and female that are needed to put the whole lap belt apparatus together. This is one advantage of a bolted in harness system it's all fixed in each at it's own mounting location and there's usually a lot more adjustability built in.