I was thinking a bit about how hot an S2+ (unattended) gets in other driver situations, and for a rough approximation, once one value is measured like done in the OP, it is quite easy to do a basic prediction:
There’s ‘Newton’s law of cooling’ that states that the amount of heat transferred from one medium to another is a lineair function of the temperature difference. This law assumes a few things that are not completely correct in our situation: it assumes a constant heat transfer coefficient (unaffected by temperature), which is only valid for heat conductance. For heat convection, which also takes place in cooling of a flashlight, the law is only approximate, and for heat radiation (also present in our case) the law does not apply. Another assumption is that the temperature is constant over the entire flashlight which evidently is not true either. Still I can imagine that this law is not completely unusable for flashlight cooling, it might just be a pretty good approximation.
Some very basic calculations for the pill temperature following this reasoning:
*The OP situation: the amount of heat produced is the same as the amount of heat transferred to the air with a temperature difference of 90 degrees minus 20 degrees is 70 degrees Celsius.
*If 4x 7135 chips are used (so 1400mA), the power of the flashlight is about half (ok, that is simplified: the battery voltage is higher, so the power is a bit more than half, but then, the led is more efficient so more power leaves the light as light instead of heat, who knows which effect wins?), so in equilibrium, the temperature difference should be 35 degrees, so the flashlight will be 55 degrees Celsius (which is the human pain threshold temperature, the body being a bit cooler than the pill makes this flashlight always pleasant to hold)
*6x 7135 chips: in between, so 73 degrees Celsius. With a little added hand-cooling, or outside with lower ambient temperature, this flashlight may probably be used continually on highest setting.
*A direct drive FET-driver, say about 6A, which is double the power of the 8x7135 driver, so 160 degrees Celsius. Ai, that is well into the danger zone, for both battery stability and unsoldering trouble. But still, in freezing conditions with airflow, the light may stay cool enough.
Some day I will check one or more of the above situations in practice to see if the calculations make any real world sense at all.