Very powerful indeed!

Though the X-Men did have a few teleporters on their side, too. It could make for an interesting showdown if Kurt and Raven tried to teleport at the same time, while one was holding on to the other.

Would the netherverse and Kurt's hell-dimension merge into one another?

I really don't think they could. And even if they did, they've have to stop ME from visiting YOU, then.

... Which would probably be easier, granted.