I used to have a mk 3 1.8GL (74 bhp) that once indicated 122mph. I gps'ed it once at 114mph when it was indicating 118, so 122 must have been about 117/118 in reality. With another 40bhp the 8v GTi must be capable of about 125-130mph I guess. My 16v mk 3 GTi has indicated 145mph a couple of times and been gps'ed at 139mph, but takes a long time to get there. If you really want to find out your cars top speed you need an empty motorway in the early hours of the morning, a gps and to be prepared not to take your foot of the floor until you're absolutely sure it ain't going to go any faster. And you've got to accept the risk to your licence, of course.