Guy starts by saying, '....When I wrote the book....'.
I hate to burst the Star Trek bubble but it is approaching the problem all wrong. The answer is found in the British (and therefore better) Blakes 7.
Distort Time.
The limitation of speed are based upon moving from one point to another in a finite time.
If you distort the passage of time then the actual velocity doesn't matter.
Increasing the passage of time, outside a field of distorted time, the actual distance travelled is increased.
Since time is a function of gravity, the objective should be to manipulate gravity.