I have an 84 jetta with a 1.6 turbo 5sp and its not really slow or underpowered. I can haul ass away from almost any car on the road and leave a great smoke trail doing it. My last tank I got 42.8 mpg and that was with my foot in it doing 80mph with the ac on everywhere I went.
My dad has a 98 jetta tdi 5sp with over 180,000 miles. He drives 55 mph coupled with really slow take offs. Hes been gettring between 52-58 mpg.
When we first got the car it only got around 49, but when we changed the timing belt we turned the egr to the offroad setting, advanced the timing at the pump, and used our computer to change the timing in the cars programming. These cars get the best mpg with the ac off, with it on the 98 only gets around 51mpg.