This is what they say Alex
During testing on a fully modified Golf Edition 30, this kit gave an incredible 44 bhp at the wheels increase when compared to the OEM intake. By allowing the turbo to maintain boost for longer, peak power was raised from 5,200 rpm up to 6,200 rpm and the engine pulled strongly all the way to the red line - gains this large won't be seen on every car, but shows what this intake is capable of.
I would be tempted for 40bhp but it seems a bit odd... i would expect maybe 15bhp. Maybe the Golf has a really restrictive airbox.