I don't know for sure. I would say, given enough road, 170mph. I got to 162 fairly quickly (in the space of about 2.5 miles) but to eek out the further 6mph took around another 4-5 miles to achieve. But that was absolute max I would ever get without going to a bigger turbo. Or down a fcuk off steep hill
Iirc Hurdy had more than 160mph in his Golf ED30 at stage 2+
The factor that you would have to contend with is the drag. Although you would get to the top speed far quicker than the Octy I don't think you would get much more out of it unless you started putting a hoooge turbo on it.