The American remake of
Inside was on the cards when I first saw the film several years ago and I really don't see it happening now -- the content needs to be so extreme it would be hard pushed to get an R rating.
As for
Martyrs, it's a film you should see as soon as possible as, if you liked
Inside and
Frontiers, you're bound to love it as it has the same extreme level of violence but takes it to another level with an unforgettable ending.
If you're interested, my review is here:
Martyrs Blu-ray Review