Hi Jonnygo,
Let's take your query a stage at a time.
1. When dropping gears a few people have noticed that pulling the left paddle once (to downshift) just before flooring it helps the car respond just a little smoother and faster.
2. I don't really use sport mode very often. When I want to drive through the gears I stick it in manual and when it's in auto I let it just cruise in D. For me S is really a bit redundant.
3. Changing gear is pretty much whenever you like. I find shifting up around 5k revs drops the car nicely into peak torque for the next gear and you can cover ground quite nicely without feeling like your straining the engine. I also find that if I'm in 3rd/4th then by 5k revs I've usually done what I needed to do.

The DSG will shift itself once it gets to the limiter (unless you have had your DSG software tweaked by companies like REVO). If you are going for straight line pace then usually just let the car shift itself at the red line.
Hope this helps.
And I managed to say all this without mentioning the launch control feature
