Righto, some updates.
Typically I would get 580 or so from a tank (80L).
The first tank since fitting only got me 510, but I may have been buggering about.
Tank 2 has just ended, and has involved my normal drive (2 x 50 miles a day, across town and then variable motorway limits and then a good normal motorway stretch before 10 mins of town driving)...and I have had some spirited moments, including 2 days of full sports mode.
Admittedly I did let the reserve go on, and using alternate sensor checks, got the tank down to (what it read was) 2L.
This got me 625 miles at an average consumption of 38.6 (was as high as 42+)
Reserve light kicked in at 580 though, which is still later than normal.
As you can see below, I grabbed for evidence. I also filled it "at the last minute" (actually read 0L for a minute, before returning to 1L) - which clearly reads low, as you can see I topped off 77L into the car for the next test. This tank is "BP Diesel Ultimate", and we'll see what that does! (yes, I got robbed on the price, but with bugger all in the tank, had not much choice)
If the 3rd tank returns the same, it's definitely improved consumption (I'm inclined to ignore the 1st tank as I was testing it out). Given I'm using the extra grunt but not otherwise driving it harder than I would normally, I would say I'm getting more power and less consumption, so far....
Watch this space...