The true answer to that question will come from actual tournament play, but in theory it handles it fairly well. Peach should be able to follow SDI to the sides. SDI down for some characters can force a hit with the wrong hitbox... that isn't completely ideal though as it means a free grab for Peach, but if you're playing fox it's much better than the alternative . SDI up should reduce the number of uair hits, by how much depends on the SDI. I'd say normally by one. Against characters that get hit more potentially more than that (especially wolf). Against fox I don't know.... he still might be screwed xP.
So yeah, to summarize when facing a good opponent SDI should reduce the string's effectiveness. However it still should work overall and it's still Peach's best source of damage.
So yeah, to summarize when facing a good opponent SDI should reduce the string's effectiveness. However it still should work overall and it's still Peach's best source of damage.