I have long liked my 18650 at 2ohm and 1.5ohm duals.... but I do stack to 6v, using the AW LiFePO RCR 123, and using my multimeter fresh off a charge it's coming at 6.5v and drops to 6.1v fairly quickly and then slowly to 5.7v.
6v is just too much for me unless I am using an atty at 3.5ohm and a VapeMate. The experience is only slightly different than my 18650s on the 1.5ohm duallies. If I had known all these results beforehand, I would've just saved the money and continued to used my 18650s and the 1.5ohm dual-
coils. -Magnus