I always boot directly into RR and skin of choice, not the desktop.
I never see the desktop in W7 or XP, and when I do get W8 I also will be booting directly into RR. Nothing will change there.
I don't need "Metro", which IMHO is butt ugly compared to a bunch of well design and thought out skins made for RR.
And as for performance with W8,....... I guess I'll see when I get my copy.