It really depends on what other mods go with these. Assuming everything else is stock, one can expect to gain a figure in the ballpark of 20HP at peak. Add a CAI, cams and a cat-back exhaust, then we might be getting up around 40HP.
I am not a tuner. If you really want an idea, take a look around the web for some duratec dyno charts for NA foci.
Thanks for the interest and the bump, but be aware that these pistons will not fit your SVT.