RC:

The 550 is already very good. To SUBSTANTIALLY improve performance further, you need to go at least for the rt12 kit with 650 HP. Cost is around 85000 EUR. Tiptronic should be slightly cheaper.

"Problem" with the rt12, besides the cost: it can be installed only at RUF factory in Pfaffenhausen, Germany. There are some exceptions but I suppose this would add even more cost to the whole thing.

 

What about RS Tuning, Christian? Apart from their customer service, I thought they were the bestSmiley.