I've read a lot on notches.
I'm going for 2.5inch hole cutter and the metal to be welded in is 4mm thick tube.
There was a chap on edition who had a nasty head on in his car which had both sides notched and it didn't show up any extra problems from the damage. Now I know that doesn't make it ok as I guess crash tests would need to be done, but providing the metal is thicker than the original and both sides are welded correctly, I can only see it as being as strong if not stronger