What I have found works for the torx bolts is to drill them. The most critical aspect of drilling and tapping is to get the drill centered. The torx inset will do that.
First select a drill bit that will just go into the torx opening and use the torx access to center it. Get that drill started to the point that it creates a depression in the center of the bottom of the torx opening.
Then switch to a small bit, something like a 1/8 or 3/16 size. Carefully drill all the way through the bolt being certain to keep aligned with the bolt.
Once you have drilled through go to the next size bigger bit, say a 1/4" and drill that. Again, all the way through the bolt. If you have an easy out, go to the first size that you can get a decent sized easy out into.
As you start to use the easy out, work the penetrating oil in to loosen the threads.
The idea is to keep the center hole as small as possible so that you keep as much meat in the bolt. That way when you twist on the easy out it doesn't expand the bolt to make it fit tighter in the hole.
If it is rusted so bad that you can't get it out with the easy out, you work your way up in bit sizes till you reach the inside of the threads of the nut. Now you can chase the threads with a tap.
You have one advantage here in that the strength of the bumper bolts is not in pulling out of the nut but pulling at 90 degrees. That means that you have leway in the strength of the threads and can lose some and still have a secure bumper mount.
Patience is your best helper when doing this. Do it right and it will work. Do it quickly and it won't.
If you do end up drilling the threads, and it is centered, you can tap it to the next size also.