While it says you're running a remap and 6-speed-gearbox, I'd at least like to point out my own struggles with fuel consumption in a recently purchased TDI90.
People call me driving like a retiree and I'm not even 30
My daily commute is about 55 miles overall (2-way), of which 37 are spent at 50-56 mph on A level roads and fairly broken motorway. I never exceed 56mph (90 kph).
Since the terrain is quite mountainous, on the way back I take a 13 mile long "shortcut" alongside the motorway that is a mixture of downhill coasting, 30mph rural towns and some 45mph inclines. Corners are taken at the highest, safely possible or permitted speed to not have to use brakes.
Using my 1.4 BBY for this route has put me in the top 3 of Spritmonitor fuel economy figures with an average over 2020's summer season of 59.8 mpg/4.72l/100km (on 205/40R17 and heavy wheels [Sline+2kg per corner]). Extras on the car are foglights, climate control and rear speakers, rear seats are left at home.
Using a TDI 90 on Sline wheels with above extras + OSS + 2x sports leather on OEM 17" wheels, I have just about managed to do the route on 67 mpg/4.21 l/100km. This just about puts me in the top 10 on spritmonitor, though at 30% the sample size of TDI90s vs 75hp petrols.
Mind that at least for me, DIS isn't really that accurate - the TDI90's actually overreads with 4.4 on the display when it's 4.2 at the pump. (So would show less mpg than actual on a UK model) On all other cars I've driven, it was the other way round with the car being much more optimistic than the pump.
On 175 winter tyres and fueling up with ARAL/BP Ultimate Diesel, I have managed to get the consumption as low as 74 mpg, but that takes an extreme effort in eco-driving and pretty much no use of the turbo boost.
Overall I, too, was a bit disappointed by the TDI90's consumption figures, especially considering that the average values on aforementioned site are not too different from each other, contrary to what is being reported on A2 specifc sites at 58 mpg for TDI75 vs. 57 mpg for TDI90s.
The only thing that I have not been able to proof myself is how much influence extras have - leather alone, at least on the rear seats, is only a 2kg difference, though I assume OSS will be quite the difference.
For a conclusion though: Yeah the TDI90 appears to use quite a bit more than a TDI75 and possibly not a whole lot less than an "empty" petrol, but the ability to "boost around" kinda makes up for it. (Though I will admit I have never driven a remapped A2 - bit of an expensive endeavour to get that TÜVed around here)