Yep mew you're right, the GTI heads have bigger ports

Drilling the hole is indeed easy, simply match the angle of the hole in the old head and drill with a small bit, then increse the size of the drill bits till it ends up the same diameter as the old one. You'll feel the drill pop into the coolant chanel.
As for the injector holes you can:
1. Leave the injectors in
2. Unscrew the inserts, drop a coin down the hole then screw em back in
3. Apparently gearbox drain plugs are the right thread