It is very clear that those since Sept 12 are not just a random sample of his sliders. Since then his sliders have been noticeably faster, about 3mph. His other pitches -- fastballs, change ups and curves -- are only about 0.2 mph faster in since then. So it does look like the new grip has resulted in a new, faster slider. Since that date he has also thrown the slider much more often.
From a nifty article by Dave Allen at Baseball Analysts. Read the whole thing. Click it. Click!