Personally I didn't mind the drifting and these ships having quick turn rates in sector space is more to do with the nature of empty space itself. When you have no resistance, because there is nothing in space, you can move things much more easily.
I'm sure though, that if they wanted they restrict slipstream turning it could be done without absolutely demolishing normal turning. The turning rate is what it is to begin with, the turning rate in slipstream is almost nonexistent. That seems to me like they have two different turning rate values. Thus, I don't see why it would require normal turning to change.