There have not been many turbo'd waterboxers, pretty sure.  Not the best
engine as a base for significant performance upgrades

   With the notoriously weak head gasket design of the WBX motor not
being especially successful holding in  even a normal fuel/air charge
sometimes, turbocharging one makes not much sense..  The 993 turbo
Porsche (a one or two model year first try by Porsche to water cool
their iconic 911 boxer motor) was very similar in design to the WBX
motor and they rarely went one whole race before blowing heads off....

  I think Calloway had it right..displacement is the one reliable way
to up power in a vehicle...when you hop something up with  turbos or
superchargers or upping the compression or porting the heads or
anything else, you decrease engine life.   Slapping a turbo on a WBX
would be asking for trouble...