You could still buy a TTRS and get 600hp from it for the same price as that rs6 though john.
*goes off for a rethink*
If you can wait till june at the latest, the kits should have been developed and on cars. We should see the first true 500hp+ TTRS in 2-3 months, the more powerful 580-600hp kit another month or two from that.
Then with some real gps verified figures you can make youre decision.
Im only going for around 530hp myself though.