I have obtained a dowpipe off a 2003 STI, the downpipe has a blanking flage at the turbo end that blanks off the wastegate, my one(2003 STI PPP) has a small pipe running from the wastegate into the downpipe.

If I fit this what effect will it have?

 

make your car slower and less responsive !

 

so why do they fit them to STI?

 

they are like that on classic impreza unless you get a aftermarket one, and are like that on non ppp newage cars wrx and sti
they restrict the flow ,proberly put them on for mass production cost cutting etc, plus they wouldn't be able to sell you the ppp upgrade if your car had the downpipe on as standard as it is a big part of it.