Just an FYI.
Got CEL on my 12 L AWD and was P0420 (bad cats) with original cats. Bought $10 Liqui Moly cat cleaner and ran it hard up a few hills and cleared code. I run regular fuel in it and have not driven it hard lately so assume this is cause. I used to go from bay area to Reno over 7,000 ft pass once a month driving hard giving it the Italian tune up. With so many miles don't really want to spend $1k on a new cat. Plan to run premium for a bit and will see how long it lasts! Will update this post if returns.
BTW this car has original brake actuator and rear air shocks (although dampers are gone with oil leaked out, at 295k miles, normal for any shock with this many miles) and over 250k miles on front air shocks and has not leaked any oil. Holds air fine.

 

Great that you got rid of the CEL, it sounds like you saved the cats.

Since you are planning to be proactive now, here is what I would do. YMMV.
Switch to premium gas permanently and use Top-Tier gas. I prefer Shell premium.
Run a bottle of Chevron Techron right before each oil change. Keep oil changes to no more than 5K miles.
Switch to Valvoline Restore and Protect 5W20. This is what I am currently using in my '07 LS460. There is enough reporting out there about its benefits in cleaning ring deposits and I want to try it.
Get a scanner that can show fuel trims, both long and short. Let us know what they are. In general you want long trims to be as close to 0 as possible, and short trims to vary between +ve and -ve.

None of the above is expensive, but requires some dilligence.

 

Just droping in. Glad to know im doing alot of that already. Using shell premium and restore and protect. I use fuel system cleaners sometimes but not as often as each oil change.

 

Fuel trims are a new one for me, this is what shows after warmed up. Will check out while driving. Also pic of oil after about 7,500 miles (due for change), about 3/4 up the dipstick so prob down about a pint. Not bad for 300k miles!