I'm in the group that would feel that as long as the modifications are of a tasteful nature or are preventative maintenance measures (upgraded oil coolers/Carrera-style chain tensioners etc) then I, as a prospective buyer would be encouraged, rather than frightened by them.
If someone started fitting Enkei 19" gold alloys and started ricing the car up, I'd run for the hills. I have developed a mathematical formula for Porsche modifications:
911s + "bling" = Resale Disaster! 