I just installed SVT seats in my 05 ST, precisely because the stock seats are c*#p.
They are like night and day! SVT seats are supportive, firm, almost infinitely adjustable for rake and lumbar support, and they just happen to look very similar to the stock ST seats.
You can find SVT seats here, and on other Focus forums for $300-$500 (or thousands if you want the euro-Recaros). Money very well spent IMHO.
CAVEAT! I have not, and I'm not sure anyone else has yet figured out how to match the complex wiring harness of the SVT seats (they come with heaters and power height adjustment) with that of the ST, in order to get the seat belt and air bag lights to go out, or whether the air bags will even work. It may be that the air bag modules between model years are incompatible.
Nonetheless, they are superb seats.