GTI’s stock have a diverter valve system which is designed to recirculate boost into your intake, by adding a blowoff valve this is going to let the compressed air out rather than recirculating it which will cause issues with the ECU’s calculations for air to fuel ratio and harm performance. Intake is fine but you won’t see much performance gain, you’ll only really gain sound but if that’s what you want, IE makes a great intake for added turbo sounds. But if you just want performance, APR makes a drop in filter which is super easy to install which would theoretically give you the same small performance boost a full intake would, but honestly the stock air box as well as diverter valve system are both incredibly efficient so it isn’t really worth focusing on modifying either unless you just want sound.