Feel i have to reply on this one. UK MIL, I can't believe that you have anything to do with GR4 as surely you should know that the GR4 can still go super and still does especially in Cyprus on ACT dets. The fairings etc have all made a difference undoubtably but the FIn still can go supersonic without to much of a drama. The fits that can go super (RtoS cleared) are 0 Fit i.e. clean with or without pylons and missiles or 10 Fit, i.e. addition of outboard stores.
The amount of rubbish I have read about this matter on this thread is amazing especially from people who profess to be knowledgable. Incidentally the jet can go super with tanks on but is not cleared!
Max speed is however limited to M1.3 (RtoS)