Old Fart, the low RPM and big props work well for the time they run. In Texaco the object is to get the best timed run to altitude from a given allocation of fuel. In the case of the 65's in things like Lanzo Bombers (85% scale weighing around 4.5 pound) we are getting a fuel allocation of between 12 and 16 mls of fuel). The object of the tuning for this event is to get the engine to run as lean as possible while still producing enough power to get the model to a good height. For my models (and my mates) this means an RPM range of between 4500 and 5000 RPM for the climb on the 15 inch props we use. The engine runs are usually in the range of between 4 and 6 minutes if we are "spot on" 6 or 7 minutes (rarely seen). Overheating doesn't occure in that time frame because the engines are usually not buried inside tight cowls and the climb to height generally keeps the engine cool.
In fact during winter especially down south we usually have a warming run prior to launch because the engines in this set up are subject to "weak extinguishion" due to the cold fuel killing the glo plug. If you can get to a SAM event in WA I am sure Paul Baartz will give you a practical demo of how this all works. I had trouble understanding it all when I first started in this discipline but it has taught me a lot about engines and a/frames. In this discipline there are "no pretty babies" only the clock is your judge (and jury) and the performance on the day is what counts.