How fast
I have seen a bunch of the yellow f18s fly. I wouldn't consider them speed demons, though they make nice scale flights.
If you want to stick to the OS91 power plant then I suggest you look at Yellow's Starfire II ARF. That's a pretty speedy plane that also slows down nicely on landing.
David is right though, if you are a speed freak and can fly well enough to handle 200 mph and over, then go looking for a BVM Aggressor II or III or a Viper or Ultra Viper. I recall these jets being clocked at 225 consistently.
Antony