I doubt very much that a single thin piece of metal can absorb and dissipate heat in any meaningful way to actually maintain or reduce temperatures. Larry Pina's Macintosh Repair & Upgrade Secrets book mentions that the SE's flyback transformer is surrounded by a high voltage shield - not a heatsink.

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Actually I made an assumption about the metal cage around the flyback being a heat sink, just as you have about other things. Thanks for correcting me on that. However, Pina also refers to a known heat sink removed from the top of early Compact analogue boards as an RFI shield, so I'm not convinced his reference is concrete proof the cage only served one purpose. As for thin piece of metal being a heatsink, the compact Macs are full of them. Take a look through Pina's book, there are a number of individual components that are attached to "irreplaceable" heatsink fins, which are no more than thin sheets of metal (in particular the Q3). I would argue your "extruded metal" sink is based on your modern understanding of a heat sink, as well as assuming the same is necessary for lower temperatures and higher tolerances of capacitors vs. CPUs. As I already pointed out, the PSU inside the HD20/SC is entirely enclosed in a metal box with no air circulation. It must be relying on a conductive heat sink to cool the internal components as that is the only surface exposed to the airflow.

Do you have something to confirm that the SE's PSU runs cooler than the exposed PSU on earlier analog boards? logic would suggest that the high number of closely positioned capacitors would restrict airflow (even with a fan), therefore contributing to an overall rise in temperature in the enclosed PSU. Also I would think they placed the PSU inside its own case in the SE for safety reasons. This would make sense, as the SE was the first compact mac to include a flyback transformer with a built-in bleeder resistor, another safety precaution that previous Macs didn't have.

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Again, I make an assumption which has not been tested. This is based on Pina's observations that the various components have a higher tolerances and therefore are not being routinely operated close to their ratings, thereby running cooler. However, Larry Pina's Macintosh Classic and SE Repair and Upgrade Secrets specifically refers to the "heat sink screw" at the front of the PSU and makes specific mention of heat sink grease if the internal insulator falls off. So clearly some aspect of the cage, if not all of it is acting as a heat sink for Pina to refer to it as such. With the Astec PSU he specifically references a heat sink as part of the assembly, as well as with the Sony hybrid PSU. In neither Book does he mention safety concerns when handling the PSU, whereas he specifically mentions discharging the bleeder-type resistor flyback "just to be safe". Also, I've never seen a reference to safety regarding dealing with the older exposed PSUs in any official publication. As for the "safer" flyback transformer being introduced with the SE, I'm not sure that was an effort only to make it safer, particularly since Pina and Apple both continued to recommend discharging them. I am also not sure it was introduced specifically for the SE which did not occur until 16 months after the first Pluses shipped. I would suggest it was simply a more robust flyback which Apple realized for some time it needed to adopt, but didn't want to pay for it. There was already a revision (if not two) to the Plus analogue board coinciding with the Platinum Plus in January 1987, perhaps to accommodate the new flyback. In any event I speculate the bleeder-type was simply a by-product of the transformer manufacturer's own innovations.

But all of this is conjecture without knowing the Apple engineer's actual rationale and doing some testing towards using the SE in a manner inconsistent with its engineering.