I've run the TK70 with 2 lipo4 26650s, 2 IMR 26650s, 2 Lipo 32700s, 5 and 6 D cell Nimhs. I checked the tailcap current and the light regulates all of these combinations so you are fine. My current readings indicate you should be able to get close to 2 hours runtime.
Personally, I would not take a chance on putting too much voltage to the driver on a $200 flashlight. It's one thing to screw around with a $25 light, it's different when the light costs almost 10x as much. What do you really have to gain, other than a strange storage spot for extra cells?
Actually I would be more concerned about the mismatch of batteries. You really ought to use 5 of the same capacity of battery, preferably in the same batch.
In any case 5xbatteries makes a huge light even bigger and makes recharging the batteries rather awkward. I think you'd lose more time recharging than the bit of extra runtime would be worth!
I can see your concern but as I stated the light regulates using two Lipo cells which is about 7.4 volts nominal. What you gain is more runtime and it isn't like the extender adds much to the TK70s considerable bulk. For me personally it provided a use for the TK60 extender and greater runtime for the TK70.
I don't blame you for not wanting to take any risk with a fairly expensive (and very nice) torch. I can only state it has been safe in my two TK70s and tailcap readings show a lower current draw overall. This and the fact that the light is no brighter indicates to my satisfaction that the driver is regulating the higher voltage fine.
