A friend wants a 03 GT
 

He is about to trade in my 96 impala for a new gt. He wants it cuz he misses the speed and the warranty that comes with a new car.
Now I know 01-02 ls1s are a bit faster than 98-00s, but I dont know about the 99+ mustangs and if they got faster over the years.

How much power do they make stock?
What 1/4 mile times will these AVERAGELY run?

Anything you guys suggest my friend replaces first? I know the exhaust, but what is the weak point in these cars? I know in my 94 it was the heads, so what about an 03 5-speed equipped mustang gt?

The new '99+ GT's make 270 horsepower (not sure about torque), run high 13's/low 14's in a 5 speed GT coupe. A guy where i work has a '01 5 spd convertible that ran 14.2 on his second time to the track, so they go pretty good stock.

The weak point in those vehicles is the T45 transmission which has a few recurring problems (but can also go for years without any problems even with abuse) and also Ford has somewhat addressed chassis stiffness, but it's still a flexy platform.

The first thing i would do if i bought a new mustang would be to get some 3.73 gears, aftermarket shifter (overshifting will break/bend the shift forks, plus stock shifter sucks), and maybe a fenderwell cold air system.

The new mustangs though are pretty well built and very fun to drive.

260hp, 302 lb-ft stock. 84LX89GT is right on as far as the 1/4 times and getting gears and a shifter. Cold air is worth nothing but looks. Lots of bolt-ons offer decent gains.
The '03 stang does not have a T45, so there are no worries with the tranny. I believe the newer ones have a Tremec TR3650 which shifts very nicely like the T45 but is tougher.
As for the flexing chassis, subframe connectors are cheap.
Your friend should take a look at the new Mach I's ;) .

I'd recommend the Mach 1 too.

DOHC motor and solid axle.

but good luck finding one :)

beware of mark-ups on the Mach I. Nice cars but not worth paying $5K over sticker.
The new GT's will make about 230 rwhp - an increase over the 5.0.
The weakest part of the new GT's IMO is the gearing. One of his first first mods should 4.10's, nothing less. A catback and X-pipe will free up some ponies.