I can't answer all of your questions but I can provide some insight. Most people skip stage one and go to stage 2. Stage 2 gives much better low end and its been said that stage 1 doesn't really "kick in" until you hit 3rd gear.
I installed stage 2 on Thursday. All I can say is holy crap. I was a tad bit skeptical at first but the car has really come alive. Its just as smooth and pleasant to drive as stock when your just driving around, but floor it and you get whiplash. I've noticed a couple of times that I've gotten the tires to squeal even with traction control engaged. I have no regrets.
I also believe that the Dinan warranty only matches the BMW 4year/50k only and not any extended warranties.