The fuel filler cap has 95 in big bold lettering from what I've seen online, so I presume 95 RON is recommended. Once I take delivery of mine, it'll be on regular unleaded, a 3-year lease car that won't see any appreciable daily performance increase isn't going to be treated to special petrol from me.
I can understand a real high-performance car benefiting from it though, like a Porsche or the Audi RS lineup, but a GTI? I doubt it'd be noticeable or worth it to be honest.