While with the addition of Slip Stream Drive I realize that this will probably never be considered I have a recommendation for increased speed while flying through Sector Space.
In Star Trek there's been many, many times where running at maximum Warp was a time-limited thing. The Star Trek TNG Technical Manual mentions that the Enterprise has an Emergency Speed of WF 9.9. There was a lot of dialog where Geordi would mention that he could only run the ship at maximum Warp for 'x' hours before damaging the engines.
Why not implement an emergency speed function in Sector Space with a time limit. It would be similar in function to sprinting while in ground combat. It could be activated using the same controls as "Full Impulse." (Give it a timeout of 30 secs like the Slip Stream drive, or use a similar "energy drain" mechanic as "reverse" to force the player to disengage Emergency Warp. OR, an engine power debuff that lasts 'x' amount of time if you use Emergency Warp too long.)
Once that's in place, let the Slip Stream drive be much faster than "Emergency Warp" with a much longer timeout.
So to summarize, Emergency Speed is something everyone has with some kind of deterrent to prevent over-use. Slip Stream Drive is much faster than Emergency Speed with a longer timeout, and only VIce Admirals have it.