Most of the 13/14 GT500's I've seen dyno average around 590-600 RWHP. I've also seen some put 570 RWHP down, so, when it all comes down to it, it depends on how generous the dyno is and how the dyno is setup.
I'd say 580-590 RWHP is a good realistic number for Stock 13/14 GT500's.
Example I'd use is 03/04 Cobras. They were rated at 390/390, however, Ford came back a short while after stating they were underrated and were actually rated at 425/425. A LOT of 03/04 Cobra's were making 360-370 RWHP, which would account for a 15% drivetrain loss, which is somewhat accurate. 2011+ GT's have seen 370-380 RWHP and they are only rated at 412HP for early model years and 420 for 13/14 GT's. That would account for an even less of a loss, more around the 10% mark on a dyno.
2014 GT - Track Pack, Leather Recaros, Electronics Package, Back-up Camera - Ported CJ Manifold, SCJ Mono-Blade TB, Custom Intake, Kooks 1-3/4" LT's, Kooks O/R H-pipe, Borla S-Types, AED Tuned - 443 RWHP / 388 RWTQ
Ordered: 3/19/2014 - Built: 5/13/2014 - Delivered: 5/19/2014