MS Office 10 for OS X is a Mach 0 Carbon application, meaning, it uses Carbon as it's primary API, but also makes calls that fall outside of the Carbon spec to better take advantage of some new OS X features (UI issues mostly).
The reason OS X's Finder is lame right now has nothing to do with it being a Carbon applilcation. It's sluggishness is due to lack of optimization. Apple got OS X running and out the door as fast as it could, and to cut corners they opted for stability over speed, and not both, as they should have aimed for. This will obviously be addressed with 10.1.
Carbon and Cocoa are both API's. One isn't faster than the other. The big benefit you are going to get with Cocoa is that your application development time should be shorter due to the development tools available. Your advantage with Carbon is that if you already have an app made for OS 9 (or OS 9 is part of your intended audience), you can make your app run on both. But just because your app is Carbon doesn't mean that it will run on both OS 9 or X. A Carbon application might have some specific calls to OS X that might prevent it from running on 9.
Hope this clears it up...
Cheers,
SerpicoLugNut
http://www.macosxcentric.com