If you're just using one 7135 chip, you're not going to lose much efficiency due to PWM if you're using a mode below 350mA. The LED is already very efficient at 350mA. However, you're already losing efficiency just because the 7135 chip "throws away" voltage above the forward-voltage of the emitter. That might be up to 25%. Still, it's better than a FET driver, though not as good as a boost-buck.
Anyway, if you plan to use your light on low a lot, stick with a single 7135 chip if that's what you're planning to use. You can probably use up to 4 chips if you want a higher max, without losing much efficiency.
All that said, on low modes you're going to get a s-load of runtime from an 18650 cell, regardless of what driver you use.
