Will give you absolutely no gain. Lighten your flywheel.
Not quite true.
The balanced (if they are) header pipes would give you a lot better flow Combined only with a ported polished and matched head and intake system. My old Mk1 Escort saw a 11bhp gain from 4 to 1 custom made race manifold and DP set up with a smoother power curve.
This was after full head rebuild and port of the 2.0 twin fast road cam engine it had fitted, which is known benefit from better flowing in std form.
Point is OP wants instant gratification which this wont give you and has a shed load of other things up front and post fitting to make it work out, providing the Ebay manifold does not crack as soon as you leather it with spongy engine mounts, hit a cooling puddle hard or find out its Chinese Stainless steel made from old newspapers and apple cores.
I personally don't like the fly wheel to light but agree it transforms the Mk3 golf like no other mod for the money.
The next best is 3.9FD and schrick Cams, but a bit more involving after that it gets a lot more expensive to make the N/A golf engines faster hence why most folks go the turbo 1.8 route.