I'd say it depends on the car, but I'd wager its closer to 20% than 15%, stock auto GXP's have been dynoed around 330-340whp, they're rated @ 415 crank from the factory. My intake/exhaust/dyno tuned GT made 332whp, its probably between 405-415 @ the flywheel.
So I think 19-20% is about right for an auto. More like 14-15% for a manual.
Oh, and my car dynoed 293whp stock...so 19%ish from the factory I guess
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned