My apologies @langsbr
for making you feel lambasted, that wasn't the intent. But the whole exhaust thing is an item that has been visited and revisited countless times over the 11y history of the e9x platform. And the fact is that you can free up lost horsepower....but for the cost/benefit you get with an exhaust, there are far more beneficial places to spend your money if you are under 500-600hp. If you are doing it just for horsepower, dollar for dollar, it doesn't make sense at this level
Think of it this way, any decent exhaust is going to be $1000-2000 (a dual 3" will most certainly be the upper end of that spectrum). What are you gaining for that $$?? If you get a 5% increase in power you would be on the high end of what people have recorded with back to back dyno measurements on this platform. So a 400hp car will net roughly 20hp increase, a 500hp car will net a 25hp increase.
Nobody argues the HP is there. But at the level of 400hp, you can bet that the person is a stock turbo car without inlets and outlets. That stock turbo car can net far more than a 20hp increase with inlets, outlets, and a tune to take advantage of that.
A 500hp car is going to be a stg1-2 aftermarket turbo. At that point you'll net more (for cheaper) with just a boost increase, ignition upgrade and a tune to maximize that.
The fact is that power (at these levels) is so cheap that it isn't financially smart to chase horsepower in the exhaust.
Once you are in the 6-700hp range things change. You are either a single turbo conversion, or some of the more wild twin turbo upgrades with all the supporting mods. You are pushing your factory engine pretty hard at this level. Do you really want to increase stresses on the engine by increasing the boost or other previously "cheaper" alternatives? At this point grabbing 30-40hp via relieving stress and reducing backpressure for that same $1000-2000 is actually smarter than any other alternative that you have left at your disposal.
All this is just talking about HP here. If you are desiring a different sound, or a different look, or a weight reduction then that changes the calculus.