I have encountered the idle problem with a Blitz DV on a local V3 Sti Type R, so it is possible, can you post a pic of the DV also do you know if it is a 2 piston type (I have never looked inside one) , do you have the OEM recirc DV, if so I would fit that and see if your idle issue goes away.
If it is the adjustable Blitz Supersound type, I would suggest adjusting it tighter (making note of where it is set now) to see if the idle gets any better.
Cheers Iain