When flying in the south east, to reduce the number of airspace warnings I have them switched off "when flying planned course".  However, on more than one occasion I have got very close to class D airspace above me without any warnings from SD.  Whilst I need to do more with my airmanship and pay attention to my height, would it be possible to have SD warn me when I'm flying above my planned altitude, particularly if heading vertically towards controlled airspace that is not in my route?