YMMV but some of us, myself included, feel that the FHN/GGB ratios are overkill. My car had a GGB when I got it that was broken (3rd gear synchro) but still drivable enough for me to put on some miles and get a feel for the gearing. The GGB was a trans I would only want if I spent every single mile on high speed interstates. Not enough gears in it for how tall they tried to run the top ratios. At moderate speeds there is too much RPM drop between gearchanges, particularly from 3rd to 4th. You'd have to wind out 3rd in order to not lug 4th and buzz the interior after changing up. The DUK has better spacing, at the cost of maybe 250 more RPM on the highway. I don't notice that difference but I do notice the more agreeable characteristics from the better spacing. Think mine is still well under 3000 rpm at 80.
To give some perspective, I live in Montana where the speed limit is almost never less than 80 and when the weather is good (and sometimes when it isn't!!!) highway traffic is always flowing at 95 mph, 100, or more. The DUK definitely does not feel undergeared even at those speeds. And when you end up on a back road with variable speeds between 45-65 where the GGB's excessive tall ratios and big gap between 3rd and 4th were annoying, the DUK is far nicer. 5th gear is also more usable, less need to downshift out of cruising gear on mountain passes etc.
Zero change in fuel mileage. The TDI engine doesn't really care whether it's turning 2500 or 2800 rpm at cruising speed in terms of BSFC from what I have seen. These are not like a gasser where lower revs and wider throttle openings make a big difference in reducing pumping losses. The only real reason for taller gearing is just for keeping engine noise down and providing more top end potential.... I am sure my car with the DUK could hit 130 or more though before it ran out of revs or power, not sure who needs more speed than that.
All that said, there is certainly a sweet spot, unless I lived on the east coasts where speeds are lower, I don't think living with a gasser trans would be easy. Being undergeared and having the engine wound up tight all the time is not much fun. I just don't think pursuing the "holy grail" and harder to find / more costly FHN/GGB is really worth it when the much more common DUK and EEN are cheaper and in many ways preferable for most driver's uses.
I got my replacement DUK from QGAP in southern CA, good service, fair price, and the gearbox has had no issues since it went in. I have bought from Frans as well and also had an excellent experience there, though I understand shipping overseas has gotten more challenging and costly than when I bought from him before the pandemic.
