mg:The new rex and sti don't have DOHC. It must have been for a reason.
my b....I thought I read another post about a sohc wrx and stimg:
Actually both the WRX and STi have always been DOHC and still are.
Now about switching a SOHC to a DOHC. Do you plan on going turbo? What kind of power are you looking for?
I would almost think a full swap would be easier and if your just looking for a bit more on the top end a good port and polish should help there.