i do remember a couple of years ago i was stuck at a bottom of a large hill and a bus packed full of passengers came down the hill skidding.... hairy moment!!
i have never heard of a aftermarket set up as of yet, but it would be an interesting project it would also need to calibrate itself the the current speedo current
Is the car still on warranty? If it is then to maintain that you need to take it to any VAT registered garage. 3rd party garages are a third of the cost of a dealer