to be honest Row, I think you're mad for even thinking about it

a lot fo turbo cars don't really need dump valves, they are very 'show off boy racer style' and not really cool for a vw.....
how ever each to their own eh!
Sound illusion's right though, short of getting a turbo conversion to your mk1 you're a bit stuffed.
There was a nasty cheap device on ebay that you could attach to your exhaust to make it sound like a dump valve but it looked like a really bad chav extra