Hey there man I used to run at the track near Barber's Point. I recently moved back to Texas. I am assuming that is where you ran your 1/4.
I think that a good driver that knows the power bands of an 05-06 GT and proper shift RPM's, (you don't just gun it to 6000 rpm's in 1st, 2nd, 3rd, etc., would easily break into low 13's with a manual or automatic trans. I dare say (without NOS, SC, etc,) simple upgrades like that of a CAI and tune, exhaust, 3.73 gears, and 93 octane and a good driver it is possible to get into a high 12.9 sec 1/4 mile, I have heard it done.
The video of the GTO and the GT that started this thread with the horribly driven stock GT and the "I have a few mods" GTO is BS, IMO.
Sure,the GTO is faster yada yada yada...but mine is a 4.6L 300 HP FORD MUSTANG stock. I don't beleive a 350-400 HP GTO is going to be me!(jOKE)No comparison IMO. I still love the stang a whole lot better, I wouldn't trade my 06 stock stang for a Souped up GTO (unless it would resell with enough money to get me a Shelby GT500