Originally Posted by Porschefile
LC wears out the clutches and the gears, using S6 wears out the gears but not the clutches. It just shifts so fast and hard that it puts unnecessary pressure on the gearbox. Try it under acceleration at high rpm and you can feel it. Similar to the fastest shift mode in SMG E46's, a few track guys have busted trans that way.
Basically it shifts up without allowing the revs to drop 100% to match the next gear. So it feels like it "surges" forward when you shift.
Personally I stick with S4 on track, still shifts very fast without bucking at all. S5 I've done before on track but the tiny decrease in shift time isn't worth the increased wear to me.
I am not sure S6 would "wear" the gears faster. Most of the gear wear happens during normal meshing when the gearbox is turning. I agree, the shifts are much harsher, so it does put additional load during a very brief moment on the gears. I assume the gears were designed for this momentary increased load, so it should not be an issue. It should have very little impact on wear.
I can see how it can be worse on a SMG. During a shift, the gears are loaded in the reverse direction while the clutch is disengaged and then they see a peak load back in the other direction when the clutch slams shut. The impact of this going back and forth on the gear backlash could start cracks in the gears and lead to low cycle fatigue. On a DCT, since the clutches slip against one another, the gears are always loaded in the same direction, minimizing the effect of backlash impact.
The higher the shift mode, the more of the engine inertia from the RPM drop is recuperated to be converted in forward thrust, hence the surge. In theory, it should translate to more efficient shifts and better acceleration. I doubt that my limited driving abilities benefit from those hundreds of a second, however I do prefer the crispness and immediacy of the S6 shifts.