As a somewhat nerdy aside, technically "top speed" is irrelevant in space too because on the ground, your top speed is the point where air resistance (and ground/internal friction for non-flying vehicles, or water resistance for boats) is equal to your engine's output. Basically at that point you can't go any faster because your engine is doing all it can to stop you from slowing down.
In space there's no air (or at least, not enough for you to care about), so the only top speed is C. The only thing that matters for maneuvering in space then is your acceleration, which is simply the force your engines put out divided by your mass. Well, I guess specific impulse matters too, that determines how fast you run out of fuel (think of it as "miles per gallon" in space). But we'll set fuel concerns aside for now.
So top speeds and even "boosting" are simply videogame mechanics that have no grounding at all in physics, though if we wanted to handwave them we could say that they are safety features built into the flight assist. The idea being they don't want you to go any faster than would allow you to turn or stop in a reasonable period of time to avoid collisions (the real reason is of course, game balance). This would also mean that what your top speed/boost speed are would actually have absolutely nothing to do with the physical characteristics of the ship, and be entirely dictated by what your ship's manufacturer considered to be safe. Although, the government mandating certain boundaries based on the ship's characteristics wouldn't be surprising.
Of course, FD doesn't give us actual acceleration values as far as I know. Which is kind of silly when you consider how trivial that would be to measure and how popular a marketing point that is for vehicles of all types IRL. However, by timing YouTube videos I can make a rough guess that the FAS goes from 0 to 400 m/s in about 3-ish seconds when boosting (this is fuzzy of course, because I'm just looking for the right circumstances to randomly pop up while someone else is flying, hardly a controlled test). This gives us a boost acceleration of around 133 m/s^2, or somewhere around 13-14 Gs. Since the FAS weighs 480 tonnes, or 480,000kg, that means that when boosting its engines have a combined thrust of 64MN (MegaNewtons, so 64 million Newtons). For reference, the first stage of the Saturn V rocket had about 34MN of thrust. So apparently the FAS has just a bit less than two Saturn V first stages strapped to it, which is pretty neat.
