This problem arises because most torx kits do not come with a #27 bit in them and a #25 is used by mistake. They must be purchased seperately.

Easiest way to eliminate this problem is to use the #27 Torx bit to remove these fasteners, while many use a #25 by mistake, it fits sloppy and usually ends up striping the head of the bolt, but for now you can use a drill that is slightly larger by a size or two than the shaft of one of the bolts you have already removed and drill the head (s) off the stripped screws. Drill only the head off and when it pops off, then move on to the next one. Once you have taken the heads off the striped ones and removed the others, the cover comes right off, and then you can usually remove the ones that you drilled the heads off with your fingers from the back end of the bolt, unscrewing it out the front as usual. This method eliminates the possibility of doing damage to the cover itself. Real easy and it works most every time. Good luck.

Very true. I just did exactly this to 2 screw heads. Mistakenly used a 25 Torx.
I used a "Grabit (http://www.4grabit.com)" screw remover for one. The other didn't respond to the "Grabit (http://www.4grabit.com)" or a set of "Easy Outs". Ended up doing exactly what Glider suggests. Then just now found this post. Great advice Glider.
Try the Grabit (http://www.4grabit.com), they're handy for lots of messed up screw heads.

I used a small chisel and a hammer. I was careful and tapped the chisel to turn the bolt.

I've also had luck useing a left hand drillbit.If the bit gets a good bite on the head it will back the screw out.

I found two good methods over the years. Automatic center punch will usually jar the screw loose enough for getting anything that fits into the screw hole and back it on out.
Should that fail, a dremel cut off tool can slot the head in seconds for a standard screw driver to back out, if your fear the tool jumping to the case, you can hold or superglue a washer over the trox screw for protection and cut into the washers open hole. These two methods have never failed me.:bigsmiley17: