Yeh, that might work but thinking about how that would be executed throws up a couple issues, like mass locking. I think it would maybe work by what you have targeted. If it's planet-bound, the super cruise will drop you just outside of mass lock range, then allow you to only use this "middle drive speed" once you're mass locked by a planet. So you're free to whiz about up to 20kms - or whatever limit they decide is best - so long as you're mass locked by a planet but the moment you exit that range you automatically enter super cruise.
This will also enable fast inter-city or inter-continental travel as well as atmospheric entry and also when leaving the planet. Obviously, not all planets have an atmosphere so the mechanic would work wholly on mass lock status.
I for one loved the rush of launching from the planet surface and hurtling into space at massive speeds. So this would be a great way to implement it. Launch, nose up, thrusters only until clear of the city/dock limits, engage "ultra-sonic drive", accelerate to 20kms, clear the planet's mass lock range, accelerate into SC and away you go