The 626 MPS would have never made it to the US anyway, first off the 626 used was the European/Japanese(Capella) model, not the US, and I believe that the 2.5 V6 used was not the US's KL but Japan's KL-ZE high-output that put out 200hp in naturally aspirated form. Plus, the MPS concept came just 2 years before the end of the 626. It was doomed from the beginning. But it did forshadow the MazdaSpeed 6.
Look at the Miata MPS concept and the Protege MPS - They became the MazdaSpeed Miata(actually the MS Miata is merely Australia's Miata SP Turbo from '01 with some different styling and wheels, still an excellent car) and MazdaSpeed Protege(an overall "meh" car, needed more power and better options).
Japan has all of the MazdaSpeed stuff down, with concept models of the 3(well, Axela), Premacy, RX-8, Demio, and I think MPV. A MazdaSpeed 3 would be ideal for the US, something that would be able to beat the SRT-4. Since a twin-turbo 2.3 is going in the 6, a single turbo 2.3 in the 3 could produce 220hp, even 200hp would be great in a car like the 3, since it's light wieght. Though I think putting that in the 3 hatch would be ideal...there's already too many tuned sedans around(SRT-4, Corolla S, etc.).