2.5" vs 3" Muffler

I finally got the chance to watch episode 9 of Motortrend's Engine Masters where a 2.5" and 3" Magnaflow muffler(straight through design)was tested.

https://www.youtube.com/watch?v=_PVXvHkr-Vs

TL;DW

Big block Chevy engine making 620 WHP tuned for open headers loses ~19 WHP with the 2.5" muffler. The 3" muffler only loses ~6 WHP, basically noticeable. The 3" appeared louder to them as well.

I believe Genesis sedans run 2.25" piping and I'm wondering if I should go with a 2.5" or 3" muffler for the 5.0 V8(2015). I'm concerned about how those two different size mufflers will affect the power on the car.

Also realized this is in the wrong section, could a mod move it to the correct one please.

One thing to consider from that video is exhaust scavenging. Those are properly designed equal length headers with a nice 4 into 1 merge. This is why it made the most power with just a collector extension, it had nothing to do with backpressure at that time.

Secondly, the 2.5" system did not appear to have the collector extension taper from 3" down to 2.5", rather just seemed to butt a 2.5" pipe right against a 3" opening. This would cause a lot of turbulence and restriction right in the collector, cannot really blame the loss on power on the muffler sizing. So naturally the 3" system will excel here as there is no choke point. I would imagine if that collector extension tapered from 3" to 2.5" the difference between the 2.5" and 3" systems would be near nill.

Before addressing the remainder of the system, I would start at the manifolds. No headers exist at this time, but the 2015 5.0s use a better manifold design than previous years (Cylinders are paired and merge into the catalyst, previous years saw all four cylinders logged together into a merge).