Wah, this very interesting thread. I only own an old baby bimmer and not much in touch with the newer model. I was just wondering that since the 523 is a de-tuned 2.5 engine, the gain potential would be presumed to be high. Anyway, the results of the mod did not surpass the 525 specs, hence, BMW themselves are still the better tuner in this respect.
I was introduced to Sepang by some friends driving japs cars, Integra, Silvia, evo & scooby etc. I go there for fun drive and experience. I managed turn 1 and 2 quite nicely and spin in circle at turn 3, do a 90 in turn 4 and by the time I reach turn 9, I'm tomato red with embarrassment (it was a rainy first trip for me). This japs car owners brought me in their cars to show me the ropes. A scooby owner took my car and did not so far as fish tail, maybe some slide due to the rain and my humble tyres. Over time, I learn that balance is really the key in negotiating the circuit, not necessary brute force.
When I get to watch the SG Trackies in action.. I was really amazed. Don't under-estimate jap cars, there are really awesome ones. I don't track as much nowadays coz me and my car is just miles behind unless it's one of those rookies trip. But I still love driving up there to watch all those fantastic machine, be it BMW or otherwise.