Yeah I was exaggerating on the $11k, but my point was that you're not looking at $30k ($24k + $6k engine mods) for a 600whp car. You're looking at $30k for an undriveable 600whp car.
To be honest, I've got a soft spot for the '03 Cobra... they're the best looking (non-classic) Mustang ever made IMO, and they're pretty impressive on paper. They're also probably pretty fun to drive what with the rear-end always chattering and sliding all over the place, but something like a GTO is going to be more comfortable, faster around the track, more practical, and cheaper.
But hell, if I was spending that sort of money on a car, I'd just spend a little more and buy an '08 STi over both of them. $2000 toy cars the Cobra and GTO are not.