I never had temp issues with my 02 M3, the water temp gauge sat just left of centre and when it was 40+deg outside it would sit in the middle. I never push my cars on hot days though. The oil temp would sit just past 100deg on hot days.
In my current 06 M3 the water temp gauge moves around a lot more, but anywhere in between the two dots is fine both on water and oil temp gauge, pushing past the second dots would not be good though.
As i mentioned in a previous post r.e. vanos, i replaced all the belts so that they were driving the water pump effectively, i replaced the fan clutch as well because that was fux0red. Its as good as its going to get.
I've read about people replacing the temp sensor resistor or something r.e. SMG failing on hot days - i am unsure about this and haven't had it done. Touch wood i haven't had my SMG play up on hot days at all - i have replaced my salmon relay as a preventative measure though.
Speaking with CortinaD put my mind at ease too as he experiences the same issues. I suppose the thing to do is not pants it when its hot, and if you are delivering a LOT of punishment to it for long stints, short shift and don't wring it out to 8000rpm in every gear.
2006 CS M3 | ESS Tune : CSL SMG tune : Eisenmann Race : Eibach : Tekarbon