i have done both, retarding, and advancing....just to play around. i only went a few degrees both ways. i dont recomend doing it. if you track to car i geuss it would be nice to advance it since your always in the upper range of the rpm range. but for DD it sucks. the engineers at bmw spent lots of time deciding where the best all around power/reliabilty would be (cam timing wise), they did a good job. so i wouldnt change it, just my 2 cents.
p.s. when i advanced it the upper end didnt have a HUGE advantage but i did notice it, wasnt worth it because i lost alot of low end.