i have them on mine, they came in the drag pack. i don't know vs stock, but i have stock exhaust except headers and everyone compliments the sound and i love it. again, i can't tell you how much power vs stock since i never drove the stock ones.
I have heard that there is minimal hp gains and are almost as much of a pain to install as longtubes which although more expensive seem to be the way to go with a nice hp increase from what I understand.
I would only do shorties if you live in a strict emissions area that will bust you for long headers. The shorties are not even legal here but they usually don't notice them.