Being that X wheels are all the same size, would S wheels fit on an X?
Are the XS wheels staggered like the S?
Are the XS wheels performance or all season like the X?

 

Yes.
No
Guys, correct me if I'm wrong, but I believe they're performance all-seasons. I have a friend with an 08 Xs and he's already gone through one winter with his all-seasons, even though I have continuously suggested a winter set up.
He had no issues last year, but now that he's got over 20K on the tires, I think he's better off with some winter tires.

 

Yes, the 09 G35xS comes with Perf. AS tires. (in Canada)
Thats what I did Aeson. The first winter was with the very, very new AS tires and they did alright. By the following winter (and 20,000km on the AS tires) I put on a winter set up of blizzaks with winter alloys. What a difference. It makes sense.
Better traction in the winter and prolonged life of my all season tires as well.