The L6 has significantly more output than your TK16 (~3500lumens vs ~1000lumens) and significantly more beam intensity (~75Kcd vs ~14Kcd). But because of the way our eyes perceive light, even a factor of 3 or 4 increase in intensity doesn’t seem like an incredible increase in brightness. This is probably the reason for your disappointment in the brightness. The 26650 cells you use could also affect the brightness. If they have too high an internal resistance it will limit the brightness.
It sounds like you want a bit more throw power, so I would recommend for you to dedome your XHP70 and increase the current. I actually would not recommend the XHP70.2. The throw with the XHP70.2 will be worse, for a given current, than the XHP70. It has a lower forward voltage so it is more efficient, and if you use a FET driver it can be very very powerful and bright, but in that case it really uses an excessive amount of current which makes it not practical for your use.
Removing the dome will increase the beam intensity by a significant amount, up to a factor of ~1.8, and narrow the beam some. With most LEDs we remove the dome with the help of a solvent such as gasoline, but with the multi-die LEDs like the XHP70 it is more difficult to use this method, though it has been done successfully. Instead we can remove the dome simply by slicing it off with a razor blade. See here for some info on increasing the throw of the L6. If you go this route note that it is important to slice the dome as close as possible to the LED die surface. If you don’t slice very close the increase in beam intensity will be less.
Changing to the smooth L6 reflector will also help with the throw, but when used with the XHP70 there will be a “donut hole” in the beam. This doesn’t make it unusable by any means, but some people are sensitive to this sort of thing.
To increase the current you could do a resistor mod as described in the link above, or you could get a FET driver like the TA driver. There are FET drivers from mtnelectronics that will work in the L6, for example this one. If you go this route you can get the D4 firmware which has a really intuitive and powerful ramping user interface. Using a FET driver will send more current to the LED in the max mode (although you can choose any brightness you want) and might increase the brightness by an additional factor of ~1.5 or so.
As far as your heat failure questions they are sort of difficult to answer both because what fails first depends on several factors, and most of us turn off the light or lower the output before something actually fails, so there is not a lot of actual data. But the components in a flashlight are pretty tough and would most likely not fail even if left on high until the 2x26650 cells ran out. As mentioned above, the light will become too hot to hold well before anything is damaged. Metal becomes too hot to hold with the bare hand at around 55C or so, which is not very high for LEDs or components on the driver.
Note, however, that when LEDs are dedomed or sliced this can leave debris or inconsistencies on the surface of the LED die which can burn and ruin the LED. So keep an eye on the LED surface for the first 10 minutes or so of being on high. A burn will show up as a black spot.