Eh, have you never heard of the STi or Evo or Focus RS or Mazdaspeed 3 or... you get the idea. None of those run huge turbos.
I'd rather a turbo'd I6 than a V8 any day. Ford made them in Australia, but they seem to refuse to put them in the Mustang. Probably because they know they'd be better than the V8 and it's cheaper to just keep banging those out.
and STi have around 260HP and are lighter (edit: STi indeed has 305), 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 psi).
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.