I've towed an 8,300 pound GVWR travel trailer for over 28,000 miles without any problem (see my photo & posts on this site) so you shouldn't have any problems. That being said, just remember that if the trailer isn't loaded correctly (about 10% to 15% of trailer weight on the tongue) the tail can wag the dog (sway) and you can rapidly loose control even with the BMW towing functionality interveening.

Regarding the transmission; use MANUAL mode. You want to keep the transmission from "hunting" gears to maintain speed. On my 2001 4.4i 5 speed the torque converter locks-up (direct drive) at about 62 mph. I use cruise control set at an indicated 70 mph in MANUAL mode 5th so the engine can be max throttled (by cruise control) without downshifting or unlocking the torque converter. Your 6 speed transmission should behave the same except the 6th speed torque converter lock-up may be at a different speed than my 5 speed.

__________________
A good pessimist is never dissapointed!