Neil,
My racing has also been in other venues and I would have to agree that how fast the temperature rises is not as important as how to maintain the optimal temperature. Having seen the data streams from a LeMans Porsche and talking to the team engineer, I can say they try very hard to keep everything at the optimal temperature while running.
How fast the temperature rises will be effected by if the case is magnesium or aluminum, have some of the ball bearings been taken out and other factors. The best scenario is to preheat the oil and control it from there. The colder the oil the more rolling resistance, and you can have several thousand movement in clearances from 50- 280 degrees. On our RMR transaxle and the Firebird, the bearings and clearances are based on 220 degree running temperatures and we will try very hard to control the temperature to keep close to that number as possible.
In your transaxle I would look at a way to preheat the oil, maybe with an external heat pad on the case and add a small pump and a oil cooler to control oil from there. To cut down on parasitic losses it is very important the amount of oil you run, I know my NASCAR friends have it down to the milliliter on their tans / rear ends. I always feel bad watching one of those races and see a trans or rear burn up and know that some one was off a few milliliters. Tony