This makes me want to put my car back to stock or find another stock stang. Either low 14s in the past don't equal low14s now, or you guys don't talk about the ones that are outrunning you. There's no excuse for a 5.0 coupe or hatchback 5 speed with headers, gears and minor bolt-ons to not be in the 13s, and there's no way a stockish GT-S is running better than high 14s without a lot of weight removal.
The only problem with 5.0s is, even if you lose to people, they still won't believe that your stock.
As far as this race goes, it's too close to call. If they 5.0 is not a vert, and the tranny isn't worn out, and it isn't out of tune, and it has a good driver, depending on the year it was made and the list goes on and on, it should have a slight advantage in a 1/4 mile race, but a disadvantage in a race to higher speeds. Chances are, though, that one of the variables is not right, and the celica has the advantage. So, short and sweet: race and find out.
Chris