Its a joke because of the way they set the car up not because of the engine. It needs better gearing in the rear end and a better trans in my opinion. I've seen them dyno as low at 330whp and as high as 370whp at the shop i used to take my power wagon to. So depending on what gear set up you get from the dealer depends on how good of a car you'll really get with the mustang and sadly they still don't offer that wide of a variety of options.
I would say 90% of the cars coming off the floor have the low gears so they can sell the "it gets good gas mileage" bit. Very few customers come in and really know what they are getting, which means they typically get a 5.0 that is just about useless unless they like making noise.
If you decide to get a mustang make sure you get the track package and a good gear ratio. Don't get shafted with street gears unless you want all noise and no go.