if the revs are hihger it may well have an air leak mate. pressure test the system.. or spray water over the inlet etc whilst running and check for leaks. you should hear it hiss [air leak] has the boost dropped ? also the car may be running richer than normal and sucking more fuel in because its getting starved and fooled by the extra air. when you find the airleak your car should run better, the idle revs will drop down to normal and should be a bit better on fuel. YOU need to put the idle screw back where it was as your not fixing the problem.. your creating another!