Except isn't the Atom 2W? Every Watt counts with a small chassis. The SSD/PCI-e would add extra power draw too which is not only a heat but a battery life issue. Then there is the price issue.
Edit: I do think that there is a market though for a higher performance device if possible, fanless and cool running in the same chassis, even if it overlaps in cost with the SP3. My wife for instance wants very small and the screen size is the minimum acceptable given wanting a decent keyboard attached. It would be very popular but I wonder if it is possible even taking out the price consideration.