I would love to see the Dyno numbers comparing a 3inch to a 4 inch on a car under 450 hp. I could bet my ass there would be no gain with a 4 inch. Too loud also. I know my 3 inch is too loud.

***Well, you have a point there. Under certain power levels, you will reach a point of diminishing returns with a larger exhaust. A turbo exhaust has to keep up with the flow capacity of the turbine. For example, a Silvia T25 cannot flow as much as a TO4E at the same pressure ratio. A 4" exhaust would create a larger increase in power with the TO4E as compared to the T25. This means that a 3" exhaust is plenty big enough for the T25. However, the 4" exhaust would not hurt low end performance in the least.