2nd gear brake boost launches are fine for the tranny, as long as you arent doing it from every stoplight. If the tranny fluid burned from the occasional brake boost, BMW would have constantly failing trannies from people driving in stop and go traffic. If youre doing a lot of racing though you should be changing your fluid more frequently, im pretty sure that semi frequent brake boosting falls well into the heavy duty use column where the recommendation is changing fluid every 30k.
Oh, and 2nd gear brake boosted launches are undoubtedly faster. If anyone gets better results in 1st its because they arent brake boosting enough or are too easy on the throttle pulling out, based on my experiences with launching this car and hundreds of 1/4 mile passes in other cars