One thing to consider: Any time you are in an instance with a station, RES or the like, you are in that instance as it moves. You stay in orbit. Our speed could all be relative. When we are flying 400 m/s, it might just be 400 m/s less than the usual orbit speed. Its not just us moving, everything is moving with us. So we could be moving and orbital velocity and are just slowing down or speeding up from that point. Even when we are at 0 m/s its just relative to where we are in relation to a planet/station/whatever. Its not just us moving, space is moving too. And we move with it.
Speed limits could easily be a safety thing, or be working in concert with the physical limitations of our engines. Its just a matter of changing your perspective. Don't look at the speed indicator in a vacuum. Take other things into account, like how the instance will stay in the same relative position to everything else in a stellar system as the objects in that system moves.
Edit:
Additionally, the earth is both rotating and orbiting at rather high speeds. 30 km/s to be precise. Yet we can fly planes and jump without the earth rotating or moving out from under us. Why? because we are moving with the earth. we stay in the same spot, relative to the earth. This all depends on your inertial frame of reference. So yes, if we use any given instance, say around a station, we are moving at 300-400 m/s relative to that station. But change your frame of reference to a planet, say earth. All of a sudden we are moving at 7660 m/s just by sitting there. Velocity indicator might be reading 0 m/s, but it is looking at a specific frame of reference.