I do not have a fuel pressure gauge fitted - though my local guru, a manifest proponent of simplicity, once said "if you really must have a panel full of gauges, the one that could be useful is a fuel pressure gauge".
That said:
* A german engineer, who seemed to know what he was talking about - though one never knows, with engineers - recently told me that the "new" fuel pump is sh!t, delivering some 0,2 less than the original Pierburg (sic!). Not sure what unit, though.
* you seem to imply there is a relation between fuel burn and fuel pressure. I heard this before, but have some trouble understanding it. Surely the carb floats will only admit sufficient fuel to float, at whatever pressure? And even if there are differences, it should be possible to compensate for them by changing carburettor needles?
* I am slightly flabbergasted about your fuel system layout. You do be talking about a Rans S6, surely? Do you have wing tanks only? Or one fuselage tank and one wing tank? The one seems as unlikely as the other, given "the return line only goes to one of the two tanks".
* As a matter of fact, why do you have a return line at all? My own pride and beauty, a high-winger also with a tank in each wing, only has lines from each tank to the header tank, each line including a shut-off valve, a filter, and (I suspect) a non-return valve at the entry of the header tank. From the header tank it is straight to the mechanical pump on the 912, with another shut-off valve and a filter on the way. No return line anywhere, I do recall Rotax recommended these at one point in time but all and sundry laughed their head off and Rotax went rather quiet then.
Mind you mine is a 912 not a 912S but I think that matters little, for this story.