OP, regardless of why the CEL is on, your car will be in limp mode, which is going to kill your fuel economy. Seriously in limp mode, expect the same fuel mileage as a Humvee. Secondly, the random BOV is probably screwing with your Air Fuel Ratio any time you come off throttle. Combine that with the pretty good chance that the previous owner cranked the boost way beyond where it should be and you wouldn't catch me driving it all that far.
Lastly, I would not trust the dealership that sold me that car and omitted all of that info to fix things the right way. From what you're saying, it needs cats, suspension work, and has engine issues - That's all expensive to fix, and easy to lie about fixing. Hell with a tactrix cable they can shut off the check engine code.
I seriously hope it all works out for you, but make sure you get everything in writing and get the repairs checked out by a different shop once they're done.