x2 on the grinder...i have grinded and drilled out alot of bolts on my jeep... saves alot of headaches when i just accepted that they are not going to be removed without breaking or stipping them...just replace with quality bolts and anti seze
Grind the head off and get the roll bar out of the way. Then penetrating oil (aerokroil, pb blaster, etc.), heat, and some form of extractor are your friends. If you can grab it with vise grips, you might have a chance. Otherwise, welding something to it is an option, or a left-handed drill bit and mabye a bolt extractor set.
I took my dremel tool and cut off both sides so I could get an adjustable wrench on them. Had to do it to 5 when I put the family style bar in mine. I sprayed them with penetrating oil, let it sit overnight, beat the tar out of them with a hammer, then they came out pretty easy with the adjustable wrench.
Whoever invented torx head bolts needs to be drug out into a field and beaten.