You can delete the rear 02 sensors and remove the cats and you replace the cats with better flow through cats you will increase your mileage and your power. Rear 02's can be deleted from the ecu and your computer won't even know they are gone. Your engine will run better and no codes.
If Chevy did things right then why in the hell did they put plastic parts on top of the engine. Hot metal and plastic not a good combination, they should have kept aluminum intake and they would have gotten better results on power and mileage. Plus the spider web under the plastic intake is...
My proportioning valve seems good till my brake line in front of the engine blew. I haven't had time to work on it, In process of replacing fuel tank at present.