Nope - two cr123a's will put 6v into the driver rather than the 3.7v the single cell lion will do. More volts = more lumens with most drivers.
They have roughly the same amount of w/hrs avail (9w/hrs for 2xCR123a's vs 9.36w/hrs for a 2600mah 18650) but you're right, a high discharge will drop the cr123a's quicker.
That depends entirely on the driver though, a lot of the drivers on the market are straight linear-regulators that put out the same amount of current regardless of the Voltage input, but I suppose that there are a fair few drivers on the market that will overdrive the LED with a higher voltage input.
![Frown :( :(](/styles/default/xenforo/vbSmilies/Normal/frown.gif)