It certainly sounds like your engine performance is abnormal. My own MK II, rebuilt just 1300 miles (and 1.5 years ago) has never had such anemic performance. Even during break in, I frequently got speeds up to 100 kph, and now she is very happy to go well beyond that. I respect the old gal's age, so I won't be doing any tests to find out of she will make the "ton" or not, but I have little doubt it could be done with minimal effort.
Wish I could offer a new angle on your issue, but at least wanted to let you know that engine has way more potential than you are experiencing.