The 17670 has a couple obvious disadvantages performance-wise comapred to 2xCR123As. Capacity and voltage. It's a misnomer to think that 2xCR123As at 1300mAh make 2600mAh, instead you get 6V at 1300mAh (2*3=6). However, the L4, having regulation circuitry that boosts lower voltage, you can make the inference that 2xCR123As have more capacity that the li-ion.
The one advantage that the 17670 has over the 2xCR123As, which makes up for some of the differences, is that it has lower internal resistance. So while it doesn't have as much voltage, the juice flows easier, more efficiently.
Surefire's nomenclature on the L4 states a 1 hour runtime on 2xCR123As,
testing shows it to be longer than that. 1 hour from a 17670 (I cannot verify this personally, just going by what other people say) is quite good. And much cheaper than constantly feeding your L4 primaries.