How fast it is depends on a lot of things including the driver. If it's a stock suspension, stock tired, stock geared car with the above mods, I would guess it couldn't break out of the 14's. The GT is heavier than the LX by a couple hundred pounds which adds a couple of tenths and if it's a stock AOD car, it's much slower.
With the right suspension, slicks, and gears, you can get a totally stock mustang into the 12's. Especially a notchback with headers, pulleys, shortbelt, electric fan, and all the tricks.
If he has say 373's and decent tires with a five speed, he might be getting into the 13's.
I've seen ported, stock head combo's like his make around 280hp, but it's impossible to be sure without putting it on a dyno. I think the best head porters around can get 300 rwhp. I recall some of the true street or trophy stock cars getting these somewhere around numbers.
You didn't state this, but if you have an FFR you already have a 1200lb advantage, which equates to roughly 120hp.
I once thought about it this way. I would have to put 5, 200lb guys in my FFR to even up the weight. That would be funny