And some aircraft are limited by airframe temperature, some by intake buzz, some by flutter, some by canopy limits, some by aerodynamic issues not associated with drag – the list goes on. It is not unusual for an aircraft to be max speed limited by a different reason in each part of its operating envelope/different config.
As far as the F3/GR1/4 goes, IIRC (been a while… Got a manual buried somewhere but can’t be r-s-d to dig it out), the ADC only calculated up to 850KTS (IAS) and the analogue ASI only read up to 800 (obviously you have TAS, but I don’t think that’s what people are getting at here), but you had the word ‘KNOTS’ to the right of it (top of instrument) and I suppose if the needle was between the N and the O then you could probably interpret that as 850.
In terms of super-cruise, many of the legacy jets would do it – but only juuuuuuuuust… And put any significant stores on it and you’re stuck under the Mach in dry (level). Useful super-cruise is the other side of transonic