I grabbed a recent copy of R&T magazine at home last night to look up the stats and compare the Focus ST (GS) to the Mazdaspeed 3 (DS) and overall they were more or less tied across the board on weight, power, performance, handling, etc. So the main difference in my eyes is that the ST has the favorable index.
Considering that in my area Chicago/Milwaukee I was battling with a solid driver last year in an MS3 in RTF, and that I usually managed to edge him on index but not raw time - then the ST probably will do quite well with the right driver IMO.