Ok JH. What you see is the result of cat temps that exceeded melt point for the slurry bond which is lining the shell. That means internal cat temp was well over 1850-2000 F for some period of time. I dont know if this is alumina-silicate bond, but it looks like it. Anyway, cat temps this high need some late burn from the cylinders, along with enough oxygen to hover around that 15:1 a/f for some time.
Gasoline problems can cause the knock sensor signal to pull timing pretty far back (I used to have printout from a supercharged 3.4 v6 that showed short periods of 50 degrees atdc!). On occasion, I would pull fuel samples from a customer complaint vehicle and then have them tested by a Federal Referee Station. Those are the folks who test and certify the really important fuels, like aviation stuff. I have seen Contaminated gasoline with 90% distillation temps approaching 400 f. I think diesel is about 440 f at 90%. This leads to high knk signal, which retards timing and throws fuel at it, along with a very long burn event at higher rpm. To put this in perspective, I run the ERC with lowest 90% distillation temp at Bonneville because I rev high, have over 14.5:1 compression, and dont like too much advance (which can push back against the crank and take away power).
A warning about knock sensing (knk). In my day, knock detection was rpm limited to the processing speed of the ecu. For example, a 4-cylinder could perform knock detection all the way to around 7000 rpm depending on bore size. The larger the bore, the slower the ringing which pushed knk high. We had three different frequency knock sensors just for our different fours, and the wrong one installed would turn on the check engine light. Heres the deal. Six cylinder even fire dropped knk detection at about 6200, and smaller v8 down as low as 4800 rpm. High rpm detonation is not a prohibited event (well..at least in my day). Now, back to gasoline.
When the vaporization is incomplete in the cylinder, some cylinders will be misfiring. At that moment it doesnt matter how rich your a/f ratio is...there is extra oxygen pumped to the cats along with extra fuel. At the substrate face you can quickly get a strong volume of around 15:1 a/f to heat the shell beyond the bond failure temp.
This entire ugly event can occur in less than an hour drive time after a bad fill-up. Nothing kills a cat like misfire and available fuel. The problem with heavy fuel misfire, is that it occurs at pretty high in the rev range, typically outside the misfire detection capability of the processors, so you dont see a blinking check engine light. Misfire detection is the only "check engine light warning" that blinks at the driver, because steady misfire is so damaging to the cat.
That heavy fuel issue is double nasty, because it makes your plug readings look lean, but puts so much heat in the bottom of the bores (where the piston speed is slow), that the piston starts leaving aluminum on the bore at the beginning of the exhaust stroke. "If the aluminum is low, the burn is slow.". Thats how you read seized pistons. Nothing seizes pistons better than late, long burn time.
Your ceramic honeycomb is pretty finely eroded, which is probably good news. When the bits are small, and make it to the subcat, they dont as often get sucked back into the cylinders on high rpm decel.
Sorry there is no way for me to offer good advice. I just dont have the know-how to make a good judgement. All I can go on, is my experience, and I have seen cats like yours in almost new cars, AFTER the customer recently filled up...and sometimes in an unfamiliar area from a generic gas station they had never visited before. Often, there would be a "hot floor" complaint, or "sticky stuff leaking under my car" complaint (melting coatings).
My " personal best technical inspection" had 194 mile on the car! It got fixed under warranty, but we had a nice interview and friendly discussion about using caution when the attendant asks if you want it in a can. The customer did think it was odd that they had to use a funnel to put it in his tank!
Time to stop, a'fore I get kicked off the bus. All that news, and no solid conclusion, but maybe itll give you some idea for your choice.
JimL
