cynic2 beat me to it. Parts of office are reportedly an unholy mess. From the description of the Mac implementation, it was running a PowerPC-specific Just In Time compiler; I read at some point that the Intel one is/was similar. I recall reading that at some point in the past, parts of Office no longer even had source code (or the tools needed to build it!), these blobs of code were just copied over from one version to the next since they worked. Of course it's the 21st century and Microsoft could use LLVM for a portable JIT compiler, but of course due to Not Invented Here syndrome that would never happen. (Plus, of course, it is true that VBA is effectively obsolete anyway.)
Anyway... yeah, I think any expectation of having reasonably complete implementations of your apps (not mine since I don't use Windows..) is a fantasy. WinRT requires a full-scale port of an app at best, and rewritten from scratch possibly. Microsoft's applications are complex and I do not think they have the level of portability required to make a WinRT port easy.
As for speed and such... sorry that's a pretty weak excuse. I don't know how bloated Office is these days, but I've ACTUALLY run OpenOffice on an ARM (1.2ghz ARM running on a Droid 2 Global, running to a remote X display.) Not only did it run but it was SNAPPY! Guess what? PDF writing support, printing support, macros, VBA, odd formatting options, and on and on -- until I *USE* those features, it's inactive code sitting on the disk (well, flash...), NOT using up CPU time or RAM. In reality, porting a huge app that is not meant to be portable is difficult, so features are dropping off.
I want an ARM notebook bad for it's superior battery life. Running Ubuntu. (Why should I put up with stripped down experience just because I'm getting away from x86? I shouldn't and i won't. The 1.2ghz ARM in my old phone benchmarked 10% slower than the 1.33ghz Atom in my netbook -- i.e. exactly the same per-clock. And it's still fast enough for a pleasant desktop experience. A newer-design, dual-core ARM? Fugghedaboutit.)