This. Use a drill bit slightly larger than the threaded shaft of the screw. Drill down into the Torx socket until the outer part of the head starts spinning, at which point you know the head has been separated from the threaded shaft, and you can separate the rack/roof parts.
If a vise grip wrecks the remaining screw stub when you try to twist it out, use a hand file to create a flat top on whatever is left of that stub. Use a center punch to make a divot as close to center of that stub as you can, then use a very small drill bit to start the hole down the center. Graduate to a whatever bit is recommended for tapping that thread size, e.g. if it’s [url=http://www.shender4.com/metric_thread_chart.htm]an M6 screw, choose a 5.25 or 5.5 mm drill bit (if you don’t have metric bits, choose the closest standard drill size). Then you can run a tap through the hole. Ideally, the tap will “find” the original female threads and just clear out the rusted bits of the screw threads.