Trouble diagnosing if O2 sensor or catalytic converter

I have an 06 T&C 3.8L and its been throwing a P0420 catalyst system efficiency below threshold bank 1 code. I have replaced both front and back O2 sensors after reading that the code could be the sensors & to try that first to see if its just the sensors. I did a big tune up while replacing the sensors as well ( new spark plugs , spark plug wires, alternator,egr,valve cover gaskets,PVC,water pump,belts) as the van had been burning more oil than I thought it should and I thought that could have caused the car to run with high pressure and rich giving the cat a hard time and messing it up. I was tying to make sure I eliminate any causes of the cats problems so that it would perk up. I took readings on front and rear O2 sensors and the front does like its supposed to fluctuate from low to high peaking at 0.75v, but the rear reading stays at 0.8v steady. The OBD tester I used said the rear should run no higher than 0.4v but I'm not sure if that info is applicable to my specific model. Please let me know what the readings should be both front & back sensors. Also I took my cat off and soaked it in dish washing fluid overnight as i had read that can loosen any grime and possibly perk it back up as well. Plus I ran a vacuum test before & after the soaking. Prior to the soak it would take a minute for the compression to go from peak after the throttle snap to the standard vacuum reading and after the soak it does better after the throttle snap it goes almost immediately back to standard reading. So the soak did help, but im not sure if my cat is still a lost cause or if its the O2 sensors that are faulty or really what to do to further check it out. The code hasn't shown up yet but the rear sensor is still at a steady 0.8v. The rear O2 did have a black char looking on the section closest to the threads holding it in if that helps to know that. I just would love to not have to buy a new cat & if something in the way the car is performing is killing the cat I don't want to slap another one on only to have the same problem down the road. Any help & advice is appreciated.