While I can understand the reference frame shift, I think FD might be missing a trick here. Supercruise could be used as the mechanism for altering your frame of reference (as hinted by the name of the drive...), with the result of getting your exit wrong leaving you in the wrong frame (with potentially disastrous consequences).
This is effectively what we're doing when we approach a station in supercruise. We get to within 1000km of the thing with our relative speed low enough and we disengage from SC. It'd be more immersive -- and clearer, I think -- if our speed was shown as relative to our target. To exit SC cleanly you need to be travelling at a low relative speed and within a specific distance.
The reason I think this would be better than the current implementation is that at the moment it's pretty jarring when you transition from one frame to another. If you're approaching a planetary ring, you see the rings suddenly stop rotating as you enter its frame. It'd be much neater (and more skilful) if you had to match relative speed with the rotation of the rings to be able to drop out correctly. If you didn't, you'd be faced with some extremely fast moving rocks!
This would also be useful for the situation where we want to land on a planet. You'd have to match your supercruise exit velocity to the rotation of the planet (and position yourself appropriately) to be in a position to rendezvous with the dock.