I have used my KL5 with the E2C adapter and a body that holds two AA's. The light comes on right away before I can turn the head down more that a turn or two.
I have not tested any run times, but it seems to be as bright as with two 123's. The AA adapter I have does not have a switch so the light is always on.
Based on experience with the KL4 the 2AA setup isn´t really working as well - I can recommend a 3AA setup which works nicely. My post is based on the similarity of the KL4 and KL5 circuit as I have no KL5. There are quite some posts on KL4 and 2AA if you want to do a search on that. The short of it is that Alkalines and Lithium AAs will fire it up, NiMh/NiCDs won´t in most cases. Alkaline can´t supply the needed current for long and using AA Lithiums would raise the question why as they are more expensive than 123´s and therefore don´t make a lot of sense driving a KL4/KL5 though.