The Mazdaspeed and STi have around 260HP and are lighter, so of course they won't need as large of a turbo.
The Evo is about in the same ballpark as the Mustang and it does have a large turbo with a lot of boost (up to 19 lbs).
The Focus RS is a 5-cylinder, so it's not comparable, but it's also not that fast of a car (0-60 in 5.9 is nothing to write home about).
It's also very possible that the 2014-2015 RS will have the same engine as this Mustang.
STi is 305.