IAS
I was led to believe the Harrier ( particularly Seajet or maybe 2+ which is better at altitude ) even carrying large weapons is good for a lot more than 450 kts, and due to the nature of the Pegasus is not so restrained by stores, it's basically a very big engine fighting drag all the time anyway and doesn't mind much what you hang on it !
More educated comment would be welcome.
I was told by an ex-Lightning Test Pilot that it suffered intake overheating at around 650 IAS, and he'd " love to fly an F-16 " as it can manage considerably faster on the clock.
Also a Rolls Royce engineer I knew reckoned the Tornado GR1 had the highest recorded low-level IAS ( distinctly different to highest speed ) at over 850 - I stand to be corrected by those who actually fly fast jets, my record was 400 or so as a passenger in a souped-up Hawk !
On the other hand, a/c such as the B-1, despite it's supposedly de-rated engines from the original spec', seem to achieve Mach .999 at low level with great ease; I've seen this done and always suspected there's a 'speed limiter' option to prevent sonic booms alerting the bad guys ?
I wonder what the actual IAS of Concorde was - surprisingly low I suspect. Damn, I used to know a Flight Engineer on them, but he only mentioned transferring fuel like mad, and before that tragically exploding Vulcans ( buried engines not being such a great thing if they throw a blade into a fuel tank ).