04 the Camaro signs off Ford pretty much wins the pony car contest by default and releases the S197 Mustang to secure the "title" if you want to call it that. 300HP old school looks technical refinements etc. you name it.
07 Everyone is happy Ford gained many new customers (read ex GM & Chrysler fans stuck w/o new vehicle options) GT500 is introduced to keep the power hungry happy further cementing the Mustang as THE Ponycar.
08-09 A slew of new models are sprinkled in to keep interest in the Mustang going while Chrysler releases the Challenger a heavywheight contender (a capable car no doubt) but nothing that the current GT500 and special models can't handle.
09-10 The Camaro will probably be released at a hefty sticker price with tons of accolades most likely due to the new chassis design thanks to GM australia and the Zeta platform, the New Mustang is introduced as well boasting updated styling and chassis/
suspension tweaks retaining the 4.6L engine with perhaps a few horses squeezed out of it due to tuning/exhaust or whatever. The Challenger will probably suffer diminishing sales due to price and the usual Chrysler's problems (drivetrain, cheap plastics in critical components see Magnum, Charger, 300C)
11-13 Camaro probably is ahead in the performance envelope but it's a pricey ride so is the Challenger IF it is still around. The Mustang will still outsell the two of them due to better management of trim levels, price and popularity. Some Shelby/SVT speical model is released with an insane amount of power and the regular GT models will probably get some new drivetrain package that will push them ahead of the two contender.