Even with the Mac 128K Apple was installing heat sinks to help pull heat away from the delicate electronics and radiate it into the case where convective air would then exhaust it. The Apple III itself was inside a giant heat sink which actually kept it cool despite being much maligned as the cause of its failure. This is why I think the PSU in the SE actually runs cooler than the exposed components on the original analogue boards. Notice even the flyback transformer in the SE is surrounded by a heat sink as well.

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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. A good heatsink that actually does its job usually has some form of extruded cross section.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. Do you have something to confirm that the SE's PSU runs cooler than the exposed PSU on earlier analog boards? Though I haven't done extensive tests in this area, 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. This is in contrast to the earlier analog boards where the components have a bit more room to "breathe". Also I would think they placed the PSU inside its own case in the SE for safety reasons - those large capacitors hold a dangerous charge, even after a unit has been powered down. 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.