The fitting is pretty straight forward. The only real problem that comes to mind is a loss in performance since the engine was designed to recirculate the air rather than blow it off. It might not be noticeable at first, but over time it will cause errors as the ecu sees less air coming in then it expected, leading to a loss in power. There's a more detailed explanation in the mk4 section, since their engines are designed for recirculating air too.