The airbox and a slight retune is worth an honest 40hp over the base hardware, it's very restrictive. The GT500KR is 540 with no other performance pieces besides the airbox...not quite the 53hp K&N is claiming, though.
I've got a soft spot in my heart for the GT500's, I worked on the powertrain development side and have been around them since early 2005, prior to them being named "Shelby". I've even ridden along in one that did 167mph (speed limiter removed) on a runway the big 3 rent for testing in Oscoda, Michigan. They're heavy cars, but certainly put a smile on your face, they make more than enough torque to be fun from stoplight to stoplight. I'm partial to the Roush P51 Mustang, though... even if they only made 151 of them and put some fugly graphics all over the body.
I'm a Chrysler dude (owned a '67 Imperial, '68 300, and currently a '68 Charger), but the performance of the new Challenger is a bit lacking- low 14's out of a 425hp automobile?
That isn't going to cut the mustard with the 2011 Shelby and 2009 Camaro on it's way, even if it's arguably the nicest looking automobile ever produced in this century.The 'Vette is better in every performance aspect imagineable, but they're still cheap, plasticy cars. I've never been impressed with a C5 or C6 interior, it feels like an ungodly fast Cavalier.
-Nick