I don't think that the 17' should be any faster than the 15". IMHO it doesn't make much sense, at least the way I view the real difference between the 17" and the 15":
The 17" is for the person who doesn't need to necessarily work on the road as much as just being able to do his work at whatever desk he finds himself at (similar to how I use my iBook both at home and at work). That way he doesn't have to get a desktop for each place.
The 15" is like the 17", only it is geared more toward being able to use it on the road.
That, I think, is the critical difference. We'll see what happens though, perhaps Apple disagrees with me. To be honest, though, I think Apple should start offering computers (at least the prosumer computers) in completely build-to-order options, á lá Dell's online store.