Laura-
If the manual says 80k, I'd go with that. If you ever run into a warranty issue, you can always point to what the book instructed you to do.
For peace of mind, you can inspect the belt without removing it from the engine. The inspection cover is underneath the main engine cover and is held in place by 2 or 3 clips. The old belt is visibly thinner than the newer one. I don't have the measurement, but maybe someone else does? With this information, you could measure it and know for sure that you have the longer life belt.
GeWilli & I replaced the original timing belt on my Golf a couple of weeks ago. It was made in Brazil in March of '01 and had the new timing belt and tensioner. In my case, the manual says 60 K change out- I have more peace of mind changing it out even though I could have probably gone 80K with no problems. This way I am covered in case of a warranty dispute. You have a better situation than I did in that you can use the 80 K interval and have the documentation to back it up.
-tom