Obviously the runtime is poor and I only have 2x AW 16340's. Could I use 2 standard unprotected rechargables 3.7v in the XT11 (I have plenty of these) What would be the advantages/disadvantages? What problems might I encounter? Why would it be a good/bad idea? I know Klarus doesnt recommend it but why?
I agree with TEEJ - 2x18350 IMR would be the best choice for a two cell setup in the XT11. Protected RCR are not really well suited in this light, due to the low cell capacity and high circuit drain.Instead of two IMR 16340, try two IMR 18350 instead. Its brighter, and runs longer.
What you are risking by going to unprotected cells is the extra safety of the protection circuit during operation. If something happened and one cell failed during operation, that could lead to a bad result quickly with unprotected cells. And of course, there is always the possibility of accidental over-discharge on unprotected cells (which then poses risks when it comes to recharging).
If you use only paired cells from the same batch, carefully monitor the health of your cells (including recharging frequently and ensuring cells always remain well balanced for voltage), and never over-discharge, then you are likely to be ok with unprotected IMR (although failures can always happen).