My measurements are at the bottom of my post here. I compared the output of a light using two dimming methods, PWMing a FET and adjustable linear constant current. The difference in efficiency is not as high as one might expect from looking at LED test curves. I.e. if you PWM a FET driven LED that gets 6A with direct drive to 3A average current, you get more than half the output at 6A. Texas_ace did a similar measurement in his test of the XHP35.
The reasoning here is that LED efficiency suffers from two main effects at high current. One is the increased temperature at high current and the other is the high current itself. When you PWM the current to the LED you get reduced efficiency due to the higher pulsed current, but the temperature won’t be as high as it is at 6A so you don’t get a big hit to the efficiency.
The exact numbers will depend on the LED and how hard it’s driven at the peak currents. 6x7135 is only 2.3A which is not high to begin with, so that’s why I would estimate the efficiency loss would not be high compared to 3x7135.