You are missing something important. 
- I was not referring to any particular cells, but all 18650’s.
- All 18650 cells are much too weak to maintain a high enough voltage for regulation at the drive currents we are discussing. That’s why regulated drivers are not made for this scenario. The context of my use of the word weak was specifically in regards to maintaining a high enough voltage for regulation. They can’t do it.
Also worth thinking about: I think your Vf estimates may be low. Both djozz and Match show much higher Vf numbers than what you mentioned.
EDIT for a little more info.
In order to maintain regulation Vin must be higher than Vout PLUS the sense voltage. Let’s call the sense voltage only 0.1v, although it’s often going to be even higher! With Efest IMR18650 2500mAh (Purple) 2014 you’re looking at an instant drop to 4v at 5A (closest graph to 4.5A per cell). Consider that you’re dropping that down to 3.9v because of the sense voltage. You’re already out of regulation and you haven’t even started!