After a few months of driving and >1,500 miles I can conclusively report that having the steel radiator expansion tank has completely eliminated coolant loss in my C-12. My engine has had a tendency to lose coolant over time and I often would have to top it off. I never observed any leakage from the plastic tank and I was using a 6psi cap to prevent stress on the tank. The engine ran quite cool but, nonetheless, there was a slow, but steady coolant loss.
This year with the new steel tank there has been absolutely no loss whatsoever and the engine runs slightly, but measurably, cooler. Now, with an ambient temperatures of <70F the engine appears to be able to maintain itself at 190-192F at 60mph with no use of the radiator fan. At higher ambient temps the fan comes on periodically.
The replacement steel expansion tank wasn't inexpensive and I wondered if I needed it; now I am convinced that it was a worthwhile change.