This is why i keep telling people to put the sensor in the manifold if its possiable.
The first restriction in the exhaust system on a single is the turbo. When all the gases merged together into the turbo and hit the turbine side, the temperatures rise a fair bit which keeps the flow good through the turbine wheel.
Tested this a while back as a customers car was seeing pretty high egts when his sensor was placed in the exhaust housing so moved to 6" from the exhaust port and it was 80c lower.
Cant see how anyone can guess how much the temps will drop in the downpipe though as different sized housings, exhaust systems and pipe diameter will change everything.
Ryan