I did the Monaro CAI with the 2-hole mod and it really increased the induction sound. I can't verify if it added any horsepower. I also made sure the airbox is sealed from any hot engine air by applying insulating adhesive backed foam in all the little nook and crannies surrounding the engine bay. Most of the air box accepts the air from the opening behind the driver side headlamp.
Unless your cone air filter is sealed with a pod or a box you are most likely experiencing hot air induction. A lot of Australian Monaro owners swear by the Monaro 2 hole CAI mod to improve the induction in their cars. I have done a lot of reading and surfing on this subject after I made a mistake of drilling multiple holes in my original air box. Now, I only have 2 holes: a big one and a REALLY big one. I am also thinking of switching the factory stock intake tube with the accordion middle into a HSV GTS solid intake tube. I think under WOT the throttle is creating so much negative pressure that the stock tube is probably "collapsing" under the pressure thereby causing more restriction. Stay away from any metal intake tube. They will just act as a heat soak and warm the air as it travels from your airbox into your throttle, which kinda defeats the purpose of the cold air induction theory. I also insulated the bottm and side portions of the airbox using fiberglass sheets purchased from Home Depot. Drive your GTO for 15 minutes, pull over, raise the hood, and feel the airbox. Hot isn't it???