I agree with Persian54. No need to use S6 unless you want to destroy the DCT sooner. S4 is fast enough and doesn't upset the car.
There is a trade off between fast shifts and slow shifts. Fast shifts are harder on the gears. Slow shifts are harder on the clutches. Gears break. Clutches wear out. In this case the DCT clutches are pretty robust. Probably will never wear out. Gears on the other hand, and the syncros, usually just break.
Learned that the hard way on my E46 M3 with SMG II.