Yeah you're right, that probably is an exaggeration as far as the 700 series goes, though I think it is accurate for the 200 which is pretty large (assuming it's not half plugged). Maybe not surprising the truck has a bigger core than the 700 even with just a small cab since its probably the same system they used in the Suburban and presumably designed it to accommodate the demands of either application...
In any case as I see it the point is still the same -- to one degree or another, if you're running the heater and it's making the cabin warm then inevitably you're looking at cooler return temps from the heater core than what's in the supply side, and if the t-stat sees that then it will respond. It's mixed with hotter return from the oil cooler and the cold start t-stat circuit so the differential is probably buffered somewhat by those, but predicting the ratios would be pretty tough because of large number of variables, e.g. how plugged is the oil cooler, heater, how high do you have the fan turned up, etc. Regardless, given the system's layout I am skeptical that it could do any good, and with the right combination of circumstances I think it could cause harm, which is why to me it doesn't seem like a great idea to use as a rule of thumb unless you have very detailed information on what's going on under the hood. This is theoretical zone reasoning, but my experience has also been that when climbing in a situation that's making the gauge move up, cranking the heater has not helped. This has been in a few different 200 and 700 series cars but not a large sample size overall so maybe others have seen different results.
745, you've had lots of chance to experiment as well. What has your experience been?
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