Q. "I'm interested in the intermediate, but very tempted by the mega cooler, what sort of mods need to be done to bonnet/firewall to fit?"

A. When installing the mega blizzard, the firewall behind the back of the supercharger will require to be reworked/pushed in to make clearance for the supercharger elbow. Once the supercharger and intercooler assembly is installed it's hardly noticeable. However to make it look factory like you can use a section of bitumen sheet around that area to make it look "factory" This can be purchased from any automotive paint shop and can easily be formed using a heat gun and is quite cheap.