Our cars have either the ACH (1985 diesels), AGS/AOP/4S (1986-1990 diesels), AWY (1990 diesels, if the Bentley can be believed - I'd lean towards 1991-1992 - it looks like a misprint), or AVX (ecodiesels) transmissions.
Us ACHers can get an upgrade by going to the AGS/AOP/4S tranny, but not much of one, and not in 5th gear. AWYers can go to ACH or AGS/AOP/4S for an upgrade. AVXers can go to any other diesel tranny for an upgrade.
However, there's an even better upgrade for lower RPMs. The ACN/AON/ASF/ACL tranny. It was used on 8V GTIs, and at 70MPH, gives a 200RPM drop from the ACH and AGS/AOP/4S, and more from AWY and AVX. This is a big deal.
This was discovered in both the TD top speed and the Jetta/Passat NA swap threads. I felt that it deserved it's own thread, however.
I think that the ACN/AON/ASF/ACL is our version of TDIClub's beloved .681 5th.
So, how much would it cost to do this swap? FWIW, I can see reasons why a GTI owner would want one of our trannies - faster acceleration, without much top speed sacrifice. So, it could be a straight across swap, and it might not be.