blimey Exactly, clean it was fine as it was designed to be a trainer. But as soon as you want to do anything with it then you would start to run out of puff. Which is a bit like F3 would above 25,000ft where any tomfoolery would need loads of burner, loads of smash or a descent during it. Until Typhoon the Brits had a habit of buying jets with limited puff (apart from Harrier that had plenty of oomph fitted around a Dragmaster 9000 body!).
The Jag was the development of Air Staff Target 362 which demanded a supersonic training aircraft. They pretty much got what Air Staff Target 362 asked for. I shudder to think how many studes we would have lost from Jag’s unforgiving high alpha characteristics if we’d used it for training. There used to be a classic video shown by IFS on how a Jag was departed at low level and so very nearly lost but for the sharp reactions of the Instructor (initials AC if I recall correctly).