Check engine light came on since work done

how many miles were on the car when you bought it? What were the condition of the catalyst assemblies when you bought it? Were the parts purchased from the dealer that did the install work or did you carry them in?

I installed a Brenspeed Stage 2 performance package that bumped my 08 Gt500 to 547 rwhp (approx 645 crank)...I had a couple of fouled plugs before and it appears it allowed enough fuel to go to the cats to burn them...last week at full boost(13.5 lbs) I had dust and ceramic granuals blowing out the exhaust... It turns out I had only about 1/4 flow through them...I now installed MRT high flow cats with an H-pipe...runs great! Just trash those stock things and get a high flow set up...( I just turn over 3600 miles) I have to say 15K sounds steep to get 605 HP...

It is possible that the cat. could be damaged and clogged. The honeycomb inside is more delicate than you'd think. A little extra heat and they crack. The downstream O2 sensor sees the change and trips the light. That usually occurs before an engine misfire; which would also trip the light.

Like Lifter583 said, just swap out the factory units for some high flow units, and monitor you're fuel mixture and boost levels (if you can).

One last thing- it's been my experience that cats are good for about 90k on a STOCK vehicle. You didn't say how many miles were on the car, so I'm just throwing it out there. I would try to contact Shelby American to find out if they sell any high flow exhaust parts.