Well here are my reasons:
1) I'm supercharged
2) Ford only went with 5W-20 in US/Can due to CAFE (I think it's called CAFE). Since from my research exact same engine in Australia is using 5W-30.
3) Under heat and hard driving conditions after some miles oil usually downgrade a little. So i'f i'm a 30 weight oil at 5K miles it's more like 20 oil.
Plus in my opinion the is not much difference between 20 and 30 oil so the engine won't feel much more stress but it will give just a little bit better protection on a dyno run or under hard driving conditions.
I originally wondered about all the people using 5w-30 or higher in their 2011 and 2012's until the track-pack engines in 2013 suggested 5w-50 for what is the same exact engine - just with additional cooling. If you're running your car hard without the oil cooler, it seems to reason that the higher weight oil would be even more important.