Hi there mate its good to have you along. The G40 engine is a real stormer - PaulGolding had one and it was a proper quick car. A 1.8T conversion would give you the best power gain and a really good tuning base.
On your exisiting car I would get a fast road cam but you will really be better off putting the money into a new engine. At least you can do suspension mods etc, without having to undo it when you get the engine swap done.