Hi shimmy its will shanes mate if bmw have changed the motor you now have a new motorsport thermostat,new water pump,oil cooler and all water pipes etc have been changed or checked .The only other parts left in the cooling system would be radiator ,heater matrix heater control valve .Its very hard to check the radiator at the track due to all the plastic cowling so feeling for a cold spot or blockage reducing flow is imposible .As some body else has said reducing air flow to the rad by following a car closley made the temp go up to 3/4 if this is true a blockage in the radiator could possibly cause a similair rise under extreme conditions such as hot ambiant temps and 100% throttle .The csl runs a closed loop 02 sytem so it is monitoring exhaust gas temp and fuel mixture all the time even at full throttle constantly adjusting air fuel mixture not to mention the knock sensor that would pull timing and add fuel if things where going lean or hot i would be very supprized if a cat would cause this issue ,if it was the cat/exhaust temps the eml would be on as the temp sensor in the exhaust down pipe would pick it up.Just my opinion hope this helps to decide what part to change first
GOOD LUCK