5v power supply from 18650s - high power, low profile
The LEDs require 5V, for a total of 17.28A. 


OK, let's round that to 100W.

One 3000mAh 18650 rated for 20A continuous current, as used in e-cigarettes, contains 11 Wh. With two, that's 22 Wh. With 100W power draw you cannot expect more than 15 minutes battery life. Factoring in converter losses, this will most likely be 10 minutes.

I assume you're okay with that, since you say "I need everything to fit in a cylinder of ~20mm". I assume you did your homework, and if you needed more than 10 minutes battery life, you would have specified an adequate size and weight for batteries. It is your problem.


  My understanding is that I can use a buck or boost converter (depending how the batteries are wired)


For this level of current, you do not strictly need a multiphase buck converter, but it would make the job easier, use smaller inductors, reduce I2R losses... Ex-National Semiconductors has several chips which will do what you want.


  and outboard bypass transistor(s) to shunt additional current around the converter (with proper heat-sinking). 


Uhhh? What?


  I am curious though, are there considerations for the buck vs boost configurations? 


At high output currents, buck converters tend to have higher efficiencies. They are also easier on the batteries, since drawing power from a higher voltage source will require less current in the batteries.