Another issue: No banking in space.
If you don't have airfoils (and air), there's no such thing as "turn mode." You can change your vector by whatever amount you like, up to your combat velocity, but no more. There are no 8G fighter plane turns in space, unless you have the power to do the same acceleration on the straightaway.
Noncombat velocity maximum depends on the density of the medium. If you have a top NCM of 1000 kph at some defined cruising altitude, it'll be less at ground level and more at a higher altitude. In space there's no limit as Nu Soard already mentioned.
Terminal velocity is the same concept as NCM max. It should be affected by the density of the atmosphere, acceleration of gravity, and the weight and shape of what's falling. Assuming drag is proportional to the square of airspeed, terminal velocity should be double four times gravity or one fourth the density of air. There would be no upper limit in vacuum. Each two levels of Density Increase should double terminal velocity.