Did you know that the 2011-2014 Subaru WRXs and STIs use the same turbocharger?  Although they’ve different nomenclatures, they are the same.  The VF48 is mounted to the STI, while the VF52 is mounted to the WRX.  The VF52 requires a different mounting flange (it makes them less than compatible where swapping between STI and WRX generations, but I believe there are adapters that allow interchangeability).

Most people think that the WRX has a smaller turbocharger that gives the WRX an edge at the drag strip.   What makes the WRX quicker is the fact that it has one less gear cog (meaning the gears may be more suited for drag strip pulls).   In quarter-mile testing, the WRX always finishes in one less shift than the STI.  It’s the same for the rush to 0-60, too.  Basically, it can go most distances with less shifting because it requires longer ratios since it is a 5-sp transmission.  It’s not quicker because it is more powerful.  The only reason it is quicker is because there’s less shifting (which saps time).  The WRX is at a 40-hp disadvantage when compared to the STI…with a 6-sp with identical ratios, the WRX will not outrun an STI.  The reason that Subaru used the 5-sp is purely for ecomony-sake (monetary, not fuel).  The added benefit is that it requires less shifts, but it does not handle powertrain upgrades all that well.  350-hp and it’s done…that’s the most you should expect out of the 5-sp before problems arise.  Can you swap in an STI gearbox?  Sure, but they’re not cheap and I don’t think that’s an easy swap…the STI gearbox is expensive, too.  That type of swap isn’t for the meek, IMO, and even if that mod is done, the car will never be an STI…there’s more to the STI than swapping it’s parts onto another car…it’s more than the sum of it’s parts.

References:

http://www.wrxtuners.com/forums/f121/my-turbo-vf39-vf52-29176/

http://forums.nasioc.com/forums/showpost.php?p=37631014&postcount=36