How do you use the launch control?
We did a race once with a modern F1 mod (we use classic mods normaly) and i was able to pass 8 cars at the start just because they had no clue how to use the launch control ehehe!
Or how to abuse it......anyways, overide launch control, dont let it go through its sequence untill the last gear. find out at what point (what gear) you need to take over in order to have maximum efficiency. depending on your setup this could be from gear 1 up to whatever.
But more importantly, each mod starts different. i dont know about the BMW mod but for example i realy hammer the tires in GP79, make them as hot as possible for a fast start but with F171 i just need to warm the brakes on formation laps and avoid friction on the tires and that gives me a real edge over those whome warm their tires.
So find out what the ideal way is tire-wise for the BMW.
And finaly, mods are wierd. ignore real life racing cars and what real drivers would do. find out what the mod does.
For example in RF3 the gearing was wierd, you would be faster short-gearing every gear appart from 5th. that was the ideal way to get fast laptimes and fast starts.
in F171 we need to gear up even earlyer then in RF3 and using only 3 gears out of 5 can make you up to a second faster per lap and with starting a very long 1st gear realy helps because the engine stalls between gears and you simply lose time gearing up.
So in order to become fast (starting or racing) its -imo- much more about finding the tricks of the mod. I would suggest taking a race replay and look at what the fast starters actualy do to be fast at starts and try and implement it in your own starts.