ive owned 2 gti's, one running roughly 240 and another well above the 330bhp mark. both had dump valves and at the end of the day i removed both and set to a recirc valve. the only thing youll have is the ptsch noise with them, with a recirc youll have lower spool times and the recirc noise sounds better IMO
i have heard that these engines dont like to be run on a dump valve but i would say the small vaume line at the top of the valve goes on the small nipple of your dump valve and the hose from the boost side should conect to your dump valve. the other one will go to the inlet tip. that needs to be blocked as this is only for the recerc valves.
I would look into it before you cange as like i said im sure the 1.8t dont like them and as said above your spool time will increase so its going to make your car slower :/