Just a little update on this fairly outdated topic. Got into it however and found out a few things:

The cruising_speed has a hardcoded limit. You can set it through the constants.ini up to a 1000 and this is what you get then. If you set it higher, you get only 990 but the acceleration gets significantly larger. The cruise_accel_time is not determining but only affecting the true acceleration time. With 2 you sure accelerate faster than with 10 (and it is pretty accurately at 10), but if your cruising_speed is not larger than 1000, you might set your acceleration time to 0.1 or even 0 and still need about 3s to accelerate to the cruising_speed. This may be true even for higher velocities; however, since they cannot be reached but are calculated to be reached within the same seconds, the ship reaches the hardcoded maximum faster, the higher the specified speed is.