That's a "loaded question" because much depends on your airframe and intended prop size. If you get a baseline reading with open exhaust, then a reading with just the header you will at least have a starting point. A 9" diameter prop on a hot .40 is like having a 2 barrel carb on a 454, it acts like a governor. I think a 8" prop will give adequate low speed thrust to get your average speed model in the air, and give the .40 a chance to unwind.