It sounds like your first set of cats is the problem... many of the early build '06 cars had to have their cats replaced becasue of failures.
If you're going to have the dealer troubleshoot it, you would be wise to have the exhaust system returned to stock - but you might try taking it in first to see - the folks at Gaudin seem to be fairly mod-friendly, and if it's the first set of cats, or otherwise something upstream from the aftermarket system, they should be able to tell.
You don't mention if you have changed the DMEs software on your car to something like GIAC or
Softronic to take advantage of the 100 octane you are using. If not, then you are wasting a substantial amount of money running straight 100 octane racing gas in your car. The stock DME is optimized for 93 octane, so any more than that, doesn't change a thing unless the software is changes to accommodate higher octane. I realize that 100 octane is available at a lot of the Rebel stations around town - if you want to boost the normal 91 octane premium, the best thing to do is to put about 3 gals of 100 in the tank, then fill the rest of the tank with 91 octane - that will get you the 93 octane the motor and software can take advantage of... anything more than 93 is a waste of your money.
brad