The 2006 WRX received a new 2.5 liter engine, similar to the one found in the 2004-2006 STi but retained the smaller TD04 size turbo found on the 2002-2005 WRX models.  These models make their boost very quick but run out of breath up top.  They still respond well to modding and really get a lot of benefit from an upgrade to an STi turbo and intercooler.