The big difference between RT and Windows 8 (regular and professional) is that they are built for different processor architectures. If we are just talking performance, the two ARM processors would certainly be able to run Windows 8 as well as they seem faster than the old Atom processors which obviously were able to run full Windows. The thing that is preventing them from it is not the processing power but the fact that regular programs are not written to run on ARM.
There is an upside to this, which always seem to be forgotten when people talk about Windows RT being useless, and that is that Windows RT is immune to every single virus and malware which is written to be run on regular x86 hardware, and the same goes for those wonderful browser toolbars. If you don't need more than what Windows RT offers, which seems to be case at the moment with my Atom tablet, the fact that Windows RT runs less programs might very well be an advantage.
The desktop is there for office, the file manager and the more capable desktop browser. I don't get it why some are so eager to get rid of the desktop on RT, as this bonus functionailty is pretty much the definitve reason for getting a Surface or 2520 instead of a regular tablet. It is a lot better to manage folders on the desktop compared to on possible metro file manager.