The theory of the collective, in the 7.3 the injectors were raised slightly in the head (not as deep in the pre-cup) and these coolant passages were plugged, for emissions purposes. These ports being plugged make the heads run hotter for a better burn, but when working the engine hard, the heat builds quicker than the limited cooling can remove. This is part of where the coolant temp spike comes from. Plus with the pre-cups in the head, and the combustion process starts there, a lot of the heat from that stays in the heads, unlike a DI diesel, where the combustion process is inside the cylinder only and more of the combustion heat is removed via the exhaust.
I've also heard of a few guys "back in the day" that would make 6.9's into "fire breathing dragon" as least for that timeframe, but wouldn't touch a 7.3. I wonder if part of that was because of cooling...... Also, it seems most having issues with cooling a turned up IDI are 7.3's but I made be wrong.......