Even if the ND is perfect and doesn't effect colour or reduce sharpness, the exact same effect could be done without an ND, simply by under exposing. If your main aim is to shoot fast glass wide open then there is a point, otherwise, all you are doing is narrowing the window of usable light before having to crank up the ISO which I really don't understand. Anyway, I don't really get your use of NDs, if you prefer shooting that way then fine but stating that they increase colour and sharpness is poor advice for anyone reading (however I assume most here are knowledgable enough to dismiss it)