Gnil:
RC:
don't know if the GT4 or GT3 RS exhaust "cut off" the sound between 30 and 70 kph but if so, it may be able to "fix" this Smiley with some sort of electrical flap control bypass and keep everything still original. No, I won't give you instructions but your mechanic may be able to figure it out, if he didn't already do.

Come on RC, spill the beans ! You will make many RS owners happy ! And we want to do it ourselves, not ask a mechanic for that ! 

You can control it if you build something like this:

http://rennlist.com/forums/991-gt3-and-rs/876050-how-to-building-a-gps-based-pse-valve-controller-for-passing-track-sound-checks...

Easiest is probably using some sort of Bluetooth relay that is being controlled manually by your smart phone or via GPS in your smart phone.