A friend did a little test a few years ago. He mounted a SRM and a PT on the same bike with a Frankencassette so that larger cogs were outboard of the smaller cogs (so the smaller cogs were inline with the smaller chainring and the larger cogs were inline with the larger chainring). I plotted the difference in the power recorded by the SRM and PT as he shifted through gears.



I've indicated the rings and cogs used for each section in the top panel. You can see that 54x16 is bracketed by 42x12 and 42x13. The middle panel shows his power output -- steady at first, then a ramp up for each gear combination. The bottom panel shows the difference between the SRM and PT power. What you can see is that more power gets through to the PT with the larger chainring, i.e., fewer losses.