Too much uplift on front at speed above 250km/h... No downforce at all... Since RS6 servotronic is already pretty light at high speed it becomes even ligher at over 250km/h.

S5 suffers from similar problems-till around 250km/h it is pretty stable but after it becomes way too light at the front. Unlike R8 both S5 and RS6 are not designed for speeds above 280km/h...

RS6 limo test mule did reach 300km/h in 51s but... Its power was little bit more then 580hp and it was a test mule after all...

Also new RS6 is a tunning horror... Engine is already termically almost at the upper limit... DO NOT TUNE new RS6. That is my advice.