Hi Stuart,
My experience with a Subaru engine in a replica is that you want the minimum amount of coolant additive possible, which depends on the annual temperature variations of your area. If you get a hard freeze, more ethylene or propylene glycol is necessary. However, maximum heat transfer occurs with plain water. All other additives decrease heat transfer efficiency. Since we are car guys, there are lots of discussions as to what type of water works best: tap, distilled, de-ionized, etc.
There are also additives that advertise additional cooling benefits, such as surfactants, that reduce water surface tension, and products such as Water Wetter, that advertise additional cooling benefits. In addition, water pumps need lubrication to prolong life, so that means another category of additive to coolants.
Air flow through the radiator, coolant speed through the engine, and square footage of rad itself, as well as rad thickness and number of coolant passes in the rad, are all factors that affect engine cooling efficiency. Cooling fan efficiency, puller vs. pusher fans, and the necessity of an efficient shroud that forces all cooling air through the rad are all elements of keeping coolant temps within range.
However, the main factor in my experience that raises coolant temp is ambient air temperature. When air temp rose over 80F, I could expect increased coolant temps. There are some operator steps you can take to reduce coolant temp, such as lowering engine rpm's, not turning on the a/c, etc., but I'm sure you are taking those steps already.
If your fans come on automatically, you may want to check to make sure they are coming on at the intended coolant temp, and, if you have two fans, that they both come on at the proper fan speed, if you have more than one fan speed. Depending on your application, there are thermostats that can be adjusted for the fans to start at a lower coolant temp, which can help keep coolant from overheating.
The crew at Beck's is not only talented, but honest as well, and committed to customer satisfaction. I can think of no other maker who will go further to help solve your problem. Please keep the group informed as you progress. That's how we all learn.