The speeds quoted are for the Harrier II (GR7/9) - about 575kts or 0.98 Mach. However I believe it can go a little bit faster accidentally
The SeaJet was faster I'm sure due to it's (slightly) more aerodynamic nose and thinner and smaller wing. The GR9 is a faster accelerating jet with the 107 engine, but once it reaches about 0.8M with bombs/tanks on, or 0.95 Mach it hits the wall and doesn't want to go much quicker.