Irrelevant. The XPS13 only has Intel graphics.

I think everyone knows the answer to that question, and it is: not yet. Future support has been announced but it's not there yet.

Theoretically it is.

Practically it segfaults or hangs X (I tried on a notebook with optimus and a computer with sperate intel onboard and nvidia cards).

Give it some time, even nvidia announced to probably support optimus with the prime/dma-buf stuff in their proprietary driver.