It is about 100 RPM difference at 80, so not a lot. And I am not sure I notice any real difference in MPGs. I only used a BEW 02J in my ALH because it was a '99.5 and I was swapping over the craptastic "wooden spoon in a bucket of marbles" shifter setup. And I had the whole trans, so I figured "why not?"
You'd have to swap in one of the taller 5th gear sets available, and unless you only drive the car at high speeds, I am not a huge fan of that. Because the 4-5 gap increases enough that I feel like I am lugging the engine too much trying to use 5th at around 45-50 MPH, so I would tend to use 4th more, in which case any fuel savings would be a wash.
But if you cruise long distances at a steady 70+ as a primary use, then yeah the taller 5th is great.