I do prefer the sound of a nice HD650 recable over stock as well, but that doesn't really say anything about HD800. Stock HD650 is pretty bleh to me anyways. I don't mind recabling in general too much, but I always first ask myself if that $200 can be parlayed into a fancier source or something. Honestly, I'd rather invest in jitter reduction before recabling: an RME or Lynx card or Squeezebox, for instance.
So it's like: "OK, I have $200-300ish to blow. I am perfectly content with my source/amp, and that money won't improve either of those at all. OK, I guess I'll try a new headphone cable then."