Exhaust bypass valve

[240SXer]

I've noticed a few people using exhaust bypass valves, which you can get a custom downpipe made where you have a valve you can open to run straight exhaust from the downpipe for more power when racing. Then keep the valve closed and run the normal exhaust when not racing (for noise reduction, etc.). I was thinking about doing somthing like this. I was curious, how much power would your average stockish SR gain from runing open header vs. 3in exhaust? I've got apex dual N1 which is pretty big already. But it was just a thought. I havent' seen it talked about here. Maybe then I'd run a Cat....