Speaking as an owner of the steptronic, you can shift into Sport mode at any time/speed. Also you can use the manual mode at any time too. The tranny will not shift into any gear that would cause it to redline (i.e. if you were going 60 and downshifted to M1, the tranny would only shift as far as it could without harming the engine). If you left it in M1 it would only shift down to 1st gear when the vehicle sensed it was safe to do so.
As for the suspension mrelussive, it is not affected by the transmission setting.