.........official limitation of the default F18 Autothrottle?....
Since, it's the default F/A-18E you are interested in, there is very little '
official' documentation to refer to. It's many years since I last flew a purely default Hornet.
But, as far as I can remember, autothrottle should be able to command throttles to any position between flight idle to max dry power.
Depending on your aircraft weight, whatever speed these throttle positions are able to achieve should be your 'official' speed mate.
This may not be much, but that's the best I can offer.
In the day's that I did fly the default Hornet, I'm certain it was able to hold speeds even as high as 550 KIAS. That's as far as I had tested I think. If the best you are able to obtain is 400 KIAS-ish, it must be something in the way your FSX/Controller is setup, I think?