Your 6AT should be fine with mod-ing. I would recommend you change your trans fluid and filter though. BMW says its "lifetime fluid" but the folks who build the trans... ZF say to change it every 45K miles. Here is a place in CA that offers alot of service kits and info on these trans...
http://www.thectsc.com/index.php?p=c...&parent=7&pg=1
Dack