Thanks Luxxy,
That would appear to be what is happening here, as I can remove the plate and revert to the normal fuel usage (so I don't think I've damaged anything or left anything undone).
I'd say my usage is much higher than yours. I'm going to put a full tank through with the 7mm plate in to see just where it is. Mine usually is around the 11l/100k for my normal weekly driving conditions.
Not sure if its placebo or not but the ute seems to pull better from low RPM with the plate in. For example when I pull out from my road onto the highway in second, it normally would shudder a little then begin pulling. With the plate in it seems to pull cleaner and better at the same speed. Its not like i'm flattening it, its more just coast up to intersection in second, then if all clear gradually roll on throttle as i get on the main road.
Seat of the pants dyno is never accurate however
Maybe hearing more turbo whistle as well. Could be placebo again.