I've done a few calculations and I don't think that the driver will ever really have to dissipate much more than 5W.
I've chosen modes of 0.2A, 1A, 3A, 7A and 10A (because they correlate easily with HKJ's graphs).
I have chosen the Efest IMR18650 2500mAh as my battery. Source: https://budgetlightforum.com/t/-/26281
And I'm using a triple XML2 for the LED voltages. Source: https://budgetlightforum.com/t/-/16621
Note that in each mode each XML2 will only see 1/3 of the amps.
At 0.2A: The battery should easily maintain 4.15v, and each XML2 will have a Vf of 2.75v. The driver will have to dissipate (4.15v - 2.75v) x 0.2A = 0.28W
At 1A: The battery will start at 4.15v but go down to 4.1v within 5mins, and each XML2 will have a Vf of 2.8v. The driver will have to dissipate 1.35W at the start, and 1.3W after 5mins
At 3A: The battery will start at 4.1v but go down to 3.95v within 5mins, and each XML2 will have a Vf of 3.05v. The driver will have to dissipate 3.15W at the start, and 2.7W after 5mins
At 7A: The battery will start at 4.0v but go down to 3.7v within 5mins, and each XML2 will have a Vf of 3.3v. The driver will have to dissipate 4.9W at the start, and 2.8W after 5mins
At 10A: The battery will start at 3.9v but go down to 3.55v within 5mins, and each XML2 will have a Vf of 3.5v. The driver will have to dissipate 4W at the start, and 0.5W after 5mins
These values are with a unprotected IMR battery (which I assume most people will use to try and squeeze out maximum amps). Using a normal battery would result in lower power dissipation in the driver. However, using a 4.35v battery will increase power dissipation in the driver a lot.
It also seems like 7A might be at the point where the driver is feeding lots of amps, but the XML2 still have a relative low Vf. If we used modes of 0.2A, 1A, 3A and 10A, we would limit the power dissipation to 4W and less. Would the driver be able to withstand this with proper heatsinking? And also, would the over-temp protection not help the situation, since limiting the driver to 3A or less would bring the power dissipation down to 3W and less.