I was in the same boat you are Spark, pretty much frustrated the hell outta me trying to get a last screw off... tried everything but kinda rushed it till it was completely stripped (the top actually came off >.<) ...
so i actually had to call in a favor from a friend who worked in an industrial panel making company.. he tried some methods, but finally settled with a power screwdriver with a fine tip (smaller then the tip of the screw). He basically made a small hole on the screw (by turning on and off the screwdriver slowly) until the tip can be fixed in the hole. Then as its fixed, he pushed down while turning on and off the screw in intervals until its a little bit into the heatsink screw hole. At this point the screw should be damaged enough to not exert too much pressure on the outside, then he basically turned it with a small wrench and it went off

yeah this method requires power tools and patience, but it saved me from what i thought was a hopeless situation. Hope it works out for ya.