I tried to buy the air filter on its own from G56 and they said no.
The G56 kit uses the stock K&N air-filter which is actually a very good unit I think and will flow very well. What MMC do though is to cover 95% of it with their stock cover and this is all part of the overall restriction in the engine performance. The G56 cover is indeed part of their kit and is in my view a great design - larger than stock so that it allows much more air in, plus a nice 'Spitfire Oil Cooler' style scoop underneath. It's been properly designed and tested and allows the engine to breath all the air it needs.
If you just want to make the most of the stock setup, it's VERY easy to trim away some (or even all) of the stock MMC air-filter cover, to reveal more of the K&N filter. This will open up the breathing very well and get you most of the way there, if you already have opened, free-flowing exhausts and don't want to buy the whole G56 kit.
Here's one I did earlier, before I got my full G56 kit;
Way cheaper than buying an S&S one and of course you can trim as much or as little off as you want. It's louder on full throttle in this format, but it will breath the best I suspect.