I have a 545I SMG. Is the car faster with traction control on or off ?

 

Hard to say. If you turn off traction control and the wheels spin, it's slower. If you leave it on and it kicks in, it's slower.

I suppose the best way is to accelerate at the limit of adhesion but how you accomplish that is the hard part...

 

A small amount of wheelspin is needed to gain the optimal standing start. This is why RaceLogic traction controls (both for street cars and F1 cars) let the wheels slip slightly when hard launched.

I would say that, for a 545i, without TC will be faster in dry conditions and a semi-competent driver. It is for my E39 M5, and that's got 400bhp to play with - obviously turning TC off in the wet or icey conditions in that is a bit of a lottery...

 

A limited amount of Wheel spin gets the engine into the powerband faster, thus faster acceleration. I would speculate that 4000 rpm is the optimum amount of wheel spin since BMW's Acceleration Assistant" launches at that RPM. Also if the car detects wheelspin with trac on, it will reduce power thus curbing acceleration.
So apply brake, DSC off for 3 seconds, floor accelerator, release brake.
Weeeeeeeeeeeeeeeeeeeeeeeeeee!