Personally I would be surprised Maemo 6 wouldn't be on the N900. Given they are porting Qt4.6 to it and will be available Q1 (in beta now). Maemo 6 depends on Qt4.6 and 4.6 has been shown running quite well on the N900. Right now the only things we know the N900 doesn't have that Maemo 6 "will support" is capactive screens with multi-touch and DRM. And I don't even know if the N900 is incapable of supporting DRM or just not support yet. This is a very different situation than the N8X0 to the N900.

As far as processor speed I don't know, but that's more of a function of battery technology than availability of faster processors.