Powering a P7 or MCE... 2x18500 -VS- 1x18650

First, looking for some clarification...

The 2x18500 setup has greater wattage output overall... right?
Are 18500 cells (generally) as severely over-rated as the smaller 16340?

Is anyone using a 2x18500 arrangement to power an MCE or P7? How do you like that setup compared to a single 18650?

I'm happy in 18650 land... but I am curious.

You'll likely land more informed hits over in the battery/ electronics section; anyway my $.02...

Not quite sure what you mean by wattage output.
2 quality 18500s will have more capacity (watt-hours) than a single 18650. Two in series will have a higher voltage output. It is generally more convenient to run an SSC P7 (or an MC-E w/ emitters in parallel) in regulation at high power from 2 LiIon cells and a buck circuit than a single Li-Ion from a buck boost circuit.

Cells being overrated have more to do with the manufacturers and the sellers than the cell size. It's true many manufacturers and sellers do overrate their RCR123's, as do many for their 18650s. 18500s are less common so there simply aren't as many people selling them and thus not as many overrating them imho. Just get a quality cell, and as usual around here AW comes to mind. You've been around long enough to know this anyway I'm sure.

Many manufacturers are using 2 18650s (or 3/4 CR123s) to power multi die lights currently. It is much more rare to see manufacturers using single 18650 setups. However many enthusiast builders and modders are doing fine with single 18650 setups. All depends on the driver circuit really.