that really is beautiful, Fred is quite the machinist! Personally I would have tried to make it one piece, with no pill, so that there’s a direct path to the outside air. Even so, the fine pitch of the threads should mitigate a lot of that and make assembling the light much easier. It’s certainly going to be a stunner when it’s done.
As for the whole thermal mass thing, I’m starting to sound like a scratched record here, but it’s pretty simple physics (simple enough for me to understand at least). The more thermal mass, the more hysteresis in the system. In other words, there will be more lag in heating up (as it can “absorb” more heat) and more lag in cooling down (as it has more heat to dissipate). For short bursts, lots of thermal mass is fine, provided you don’t saturate it. This will help keep the LED cooler for those short bursts. More thermal mass = more time before it’s saturated/ at equilibrium for a given thermal input. A pound of copper would most likely take longer to heat up (and it still has a decent surface area anyway) than most torch batteries would run for.
For sustained use (ie. once the LED and housing reach a steady state temperature), the ability of the housing to dissipate the heat generated (which is the result of the thermal resistance of the heat path and the surface area of the housing, plus the difference between housing and ambient temperatures) is more important. The fact that most of your high power torches cannot be left on for an hour or more without overheating (ie. housing temperature does not stabilise or will only stabilise at a temperature way higher than is safe) demonstrates clearly that the housing is unable to dissipate the heat generated. Adding more mass to the pill isn’t going to make the blindest bit of difference, it’ll just delay slightly how long it takes for the housing temperature to start rising. Even directly bonding the LED to the pill isn’t going to help if the housing can’t dissipate the heat.
As for P60 drop ins vs. threaded pills, the argument that you can create a thermal path between a P60 dropin and the housing isn’t evidence that the thermal resistance of that junction is lower than a threaded pill installed with the same care (ie. using thermal paste), it just means you’ve done a good job working around the disadvantages of the P60 format. As long as the pitch is fine enough and the threads fit well, you’ll have far more metal to metal contact (excluding the thermal past filling any gaps) than with a P60 pill - otherwise your pill will just rattle around and come loose, right?