Rad,
400 fwhp in stock configuration is attainable, but it would naturally require an upgraded fuel system and a very good tune. I would personally go turbo, ok I'm bias, but the ability to turn the boost up and down depending on the gas (standard pump vs race) and not having to spin the motor to high rpms to produce boost are both bonuses in my book.
To get 400 rwhp would require some porting on the factory heads and intake, but is easily done.
As far as Pro Turbo, yes they do promise a 21 day delivery, but no one has yet to see one of their kits on a car and making hp. The information they have provided does lead me to believe they are going to be a good company to deal with and someone has to be the first, why not you right.
Check out
www.turbomustangs.com for alot of information and links to about everything turbo related you might want.
Also the guy running Pro Turbo does post on there so you should be able to get as much info as you want.
Might I also suggest Turbo Technology, they are well know for getting their kits out quickly and are none for having some of the easiest kits to work around once in the car, unlike some of the others.
Good luck with your decision.