While a turbocharged BRZ may make itself into the mainstream market - it definitely won't be for 2014.
A) Subaru spent a lot of money on the project and it doesn't make sense to go around and change everything a year after production... They'll want to milk the platform they have now for as much as they can... Especially since they're limiting production this year.
B) The BRZ is having issues that Subaru will want to work out before developing a different power train.
C) The BRZ transmission is actually not designed to handle the power most people are wanting from a turbo BRZ which means they literally have to redo everything.
D) That redesigned drive train will cost more money for consumers. Which like Chano said is kinda going against the whole idea of the car anyway.
E) The Subaru reps I've talked to about a turbo version have told me not to expect one for at least a few years if they decide to actually start to develop a turbo BRZ. They've confirmed Subaru is considering the idea but has not determined whether or not to move forward with it.