Hmmm, interesting. It sounds promising in theory; colder intake charge = denser intake charge = bigger bangs = more power. One word of warning though, I have come across a slight practical problem when applying this to my Mini. Now admittedly the Min' does still use a stone age S.U. carb that doesn't even know what day it is compared to the Golf's management system but we're still basically talking fuel and air here. Originally the Mini was fitted with an inlet manifold which was cast in unit with the exhaust, thus giving rather good heating. Mine now has a tubular exhaust manifold and separate inlet, the latter having provision for water heating to replace the lost exhaust heat. Now I, thinking along the same lines as you, eshewed such things and even pushed on further by arranging a cold air feed from the grille. Result, good power but lousy low speed throttle response. Everything was just too cold and the fuel was falling out of suspension in the airstream. Connected some water heating and lovely snappy Mini throttle response came back. Not saying the Golf will do this but just bear it in mind.
S.M.