I’m wary of screw extractors. They’re quite brittle and if they snap before the screw comes out your trouble has immediately multiplied.
I’ve never snapped a Torx yet.
Case like this, I’d be about evenly tempted by trying a Torx or simply reaching for a drill.
Good thing about Allen screws is that they are by nature self-centering when you go to drill the head off. And the steel for a top cap bolt is relatively easy to drill. Run a 5mm drill down there and the head of the screw will come off. That should pop the top cap off, which should allow you to reach in there with a set of pliers and unscrew the rest.