I put 3 inch on mine and could tell a difference but others test without pictures may not tell the whole story. If you come out of the turbo and instantly go huge this is bad. if you transition a pipe from 3" to 2 1/2" instantly this is bad. Don't know what Dave Buschur did but not to may places sell cone shaped pipe and this part is easily over looked because of the masses selling transitional reducers
Now on my car it comes out at 2.3" or something id on the turbo I found an elbow and welded to a v band clamp for the schwitzer then gradually increased the size by making a long cone and welding together then in had an elbow welded to the IC, air passed through the core and then out another 3" elbow that was welded to the core. Then up past the radiator into the pipe on the throttle body that transitioned from 2.5 inch to 3 inches over about 8 inches. At first it had a silicone reducer and when i rebuilt the top pipe the car lost like 4 pounds of boost without changing a thing so this was a major bottle neck. 3 inch IC pipes if done correctly can show a large benefit I'm sure as long as you have a turbo large enough to handle it. You just have to remember air has mass and flows more like water than what people think,the statement It will be fine is used to much for fluid dynamics, as long as the flow is laminar and the flow doesn't go turbulent benefits can be made but fast transition couplers should never be used. The atoms on the side of your IC pipe are locked on there because of the friction facto/roughness of the pipe then the air molecules pass over each other in sheer into a cone shape point.
For example the picture below is one that I built, its in the for sale section now. The air as it changes from 3 to 2.5 gradually reduces to increase the velocity and fill the entire pipe instead of going turbulent in a reducer and only filling a small part. You have to imagine a cone flowing through the pipe if you hit a 1/2 inch tall wall around the whole edge the air that is traveling super sonic in the pipe now smacks the wall and angles inward going turbulent and blocking flow. a 3 inch to 2.5 fast reducer may coke bottle the shape of the airstream for a few inches but thats still a reduction of flow in the system and may make it worst that what you originally had. I could preach on this for hours stuff fascinates me. But my reply go for it it does improve performance if you a have
(a) turbo large enough to support it and
(b) do it correctly
Its not that hard to make a cone cut the pipe down the middle and then use a box of worm clamps or if using steel you'll be needing T bolt clamps. clamp it down letting one side ride over the other and when you have correct shape and size mark it with a sharpie and remove clamps then cut out the wedge/pie shape reinstall the clamps and then stitch weld.
just google research turbulent flow, laminar flow, relative roughness, friction factor, reynolds number, cone reducers,turbulent flow in orifice... pictures help so just google and click the image link at the top
