Good plan! Especially when there are no O2 sensor codes. 
Tester
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The CA/NY-spec cats will probably last longer before needing replacement. If you are planning on keeping your G6 on the road for many tens of thousands of miles & quite a few more years, you’ll be money ahead to just install the better cats now. I’m also doubtful (as the poster above seems to be suggesting) of your “replace the O2 sensors” idea, absent any experimental proof they are faulty. You might argue that a diy’er replacement isn’t overly expensive, which may be true, but the problemis that you may be replacing good sensors with iffy sensors, creating more problems than you are solving. OEM O2 sensors are pretty robust, and suggest to use only OEM sensors if you still decide to replace them.
Haha. Well, the second garage insisted on replacing them, the car is a 2008 with nearly 100K miles, and $70 for an O2 sensor that I can replace myself beats ~$900 to replace the cat, in case we get lucky.
That occurred to me, and thanks for reinforcing that point. I hadn’t dug deep enough yet to look at warranties. Was leaning towards finding an OEM cat somewhere because most everybody says they last longer than an aftermaket. (And my friend will be very happy when things stop breaking and he can enjoy driving the car again.) If a CA/NY aftermarket cat approximates an OEM, that could very well be our answer.
I had a cat failure on my g6. Replaced by the dealer under warranty. The way they diagnosed it was to clear the code and if it came back to replace the cat. I’ve put over 100,000 miles on it and the original cats and no problems. Like they say, cats usually don’t wear out. I will take the 5th on why the first one went out.
If it needs a cat, I had great luck going through Car Quest/ADV using the MagnaFlow Direct-Fit Catalytic Converters, they are the closest thing to OEM that I can remember and you can get them California Grade CARB Compliant…
Maybe the experts here know if the cali compliant ones are any different or better than the standard or if they are like the lifetime mufflers? Same product just longer warranties.
Just wondering. I understand having to pass inspections, but just wondering.
FYI, we’re going to try some homegrown cleaning/diagnostic techniques on Friday or Saturday. If we’re committed to replacing the cat anyway, we can’t really do any harm. Will report back.
Big news! At least for the present, the cat problem is solved…and for roughly $20! 
Compare to the Bank 1 test above:
In a YouTube video on cleaning cats, in the comments someone mentioned removing the upstream oxygen sensor and carefully spraying an entire can of throttle body cleaner into the hole while the engine runs. I then cleared the codes and waited for the results. Today the readiness indicator came back and with no P420. Going to get it to the garage for inspection ASAP.
Comparing the two cat tests, you’ll notice that the Min value changed! I still want to know what these numbers mean, and the longer I’m stuck fixing this car, the more I want to see the GM Service Manual. But in either case, the new Test Value is in the thick part of the range.
If I had to do it again, I would have let the car run about 5 minutes before spraying. There are YouTube videos of people actually removing the cat and soaking it in a solvent for a few hours, but with minimal results, It occurred to me that the difference here is the cat is HOT…which makes the attached gunk that much more pliable. (Removing the cat wouldn’t have worked easily anyway, because while some disconnect to a milk jug-sized unit, this cat is welded to a long pipe.) Thankfully even my amateur efforts worked.
Good for you. Suggest to mark your post above as the “solution”. The “Solution” Icon is at the bottom, only you (as the OP) can see it. Keep us informed how long this works.