Sorry Coff, missed my call! (Been busy)
As others have said, a clean GR4 is supersonic capable. The big tanks are 600kts/M1.2 capable (and more if you ignore the RTS!!) and the little ones are subsonic only (again, I know they will go supersonic!). The problem with the GRs is that they start to hit a brick wall around 1.2M without the ramps being enabled - they are fitted for but not with (that said, from the F3 'reduce to produce' there would be plenty of spares).
The good old Tonka will make it up above 40k but doesn't like it when you turn as the wing loading is too high and the RB199 was optimised for low level.
LJ