What you will need to do to fix this problem is raise the idle to 950RPM. You can do this if you have a handheld tuner, which i am assuming you do because of the downpipe. You will also have to have the secondary o2 sensor deactivated or you will get a CEL.
The reason that this happens is because you removed some backpressure, just enough for a drop of oil to leak from the rear shaft seal of the turbo. Just one drop will create the smokeshow that you are seeing.