The tuner you need for the 2006 300C Hemi is Diablosport Predator U7135 or Superchips 3865 or Superchips Cortex 3950
The Diablosport has more customization available for the timing & fuel maps, the Flashpaq is the least expensive and the Cortex includes a 2 year powertrain warranty: otherwise the tuners are pretty much equal with about the same horsepower gains and the same features.
The Predator customization can be very useful if you are modifying things like intake, exhaust, cam, injectors etc. If you're not planning any of these modifications, then the Flashpaq will save you about $25 or the Cortex can give you some peace of mind with a 2 year powertrain warranty.
Unfortunately, none of the available handheld flash tuners will disable the rear O2 sensors which is what I think you meant by eliminating the CAT. That feature is available in some Predators, mainly for the Ford vehicles, but none that I can find will do so for the 300C.