I finally got around to looking at the EZ30R in my garage to see if it's got a factory oil cooler. The engine is still in it's crate, so I didn't get a good look (had to use a mirror to see where the filter sits) but it does, in fact, have an oil-to-water cooler, just like the ones on the EJ engines.
An external oil-to-air cooler will be even more effective, assuming it's sized properly and you can get good airflow to the cooler. I added one of the factory oil coolers (like the Forester kit you linked above) to my EJ25 and it dropped oil temps about 10C on the track. I would expect with a good external oil cooler that I could get that down another 10 or 20C.