Yes it is unburnt fuel which will probably occur at the higher rev range when the car is being driven hard.
This is not ideal as bhp is lost, but on the other hand it serves as a way to avoid detonation as the extra fuel cools the combustion temperature.
All Evo's are generally mapped as std to run rich,doesn't help economy,
but generally to safeguard the engine, from fuel cuts during cold weather,
when boost is generally stronger thus needing more fuel, and in servere cases
to prevent detonation.
Therefore black smoke from your exhaust is generally acceptable, grey smoke inicates a problem.
Programmable or mapped ecu's generally cure this problem by optimising the fuel to air mixture.
This is why your exhaust always seem to give out a lot of fuel vapours.