Got the Mosselman extension kit installed and plumbed in series with my existing cooler. I neglected to take a single picture since I was in a big time crunch but basically the oil goes from the OFH to the main center cooler, then to the second cooler in the driver's side duct, then back to the OFH.
In this config, with ambient temps around 90F I was able to push as hard as I wanted for as long as I wanted, at Watkins Glen, on slicks. Oil temps were 250-260 consistently, and a couple corners of lift and coast would bring them down very quickly.
This still seems really high but our temperature sensor is in the sump so it's measuring the oil that has just passed through the engine, not the oil flowing to the engine like other platforms... I think 250 sump temp is probably pretty good.
The really surprising thing and what was the most encouraging is this extra oil cooler seems to have kept water temps ~10F lower compared to the nice cool 70F track day above. This tells me the oil passing through the engine is substantially cooler than before.
I'm really impressed how big of a difference a fairly small oil cooler made. This mod could be applied to the M3 chassis, utilizing
Mosselman's kit,
1M duct, and
AN adapters for the Oil Return Pipes you'll just need to cut vents in the wheel liner. Chuck their AN8 hose and cooler adapters and get AN10 versions.
The only real downside is you need to remove the lower snorkle from the intake box, and slightly trim the bottom port but this is how my car has always been since the snorkel doesn't fit anything in the 135 bumper.