For a hard launch, you want to get as much power to the ground as possible in the shortest period of time. In the 335, rev to ~2k in 1st and let the clutch out more abruptly than usual. Don't dump it, but let it out quick so you aren't slipping it. Tires shouldn't complain too much, maybe a chirp.
You may not win a drag race with that method, but it's a safe way to launch the car quickly with minimal stress on the drivetrain.
If you're going to turn DSC and DTC off ..... do it on a wiiiide open area, like an empty parking lot. You are on your own when you do that (BE CAREFUL). I would just turn DTC off to start with (one press). It will allow the tires to spin but not allow much side-to-side behavior (e.g. drifting).
6K is the right shift point as noted.
Launching in 2nd will put more stress on the drivetrain.
And please again be careful.
2011 BMW E92 335i, 6MT, Le Mans blue - Quaife LSD, JRZ RS Pro, JRZ camber plates, Eibach ERS, full M3 susp, custom toe arms, Powergrid end-links, SS brake lines, brake ducting, Michelin PSS, UUC SSK, ETS FMIC, ER CP, Perf. Exhaust, VAC RSIK, Sparcos; work by VAC Motorsports in Philadelphia