The GR1 was designed, I think, with supersonic intakes which included hydraulic ramps which influenced the airflow into the engines at high speed. AFAIR, the 'RAMPS' were inhibited from the very early days. We used to do a supersonic run at TTTE (clean jets, not even pylons) but once you put tanks, ECM, CBLS or real weapons on there was not much need for supersonic ramps (bloody thing wanted to M1+ at Goose aand Nellis though
)
Supersonic intakes and the hydrailic ramp mechanisms seemed like a lot of unnecessary weight to carry round but it was probably more expensive to replace them.