The ZZP doesn't have that step down in their 2.2 DP
Their pipe seems to step down to two inches at the rear flange
They also mandrel bend their tubing, the catalytic converter is smaller.
So, we shall see, I believe the 2 1/2 pipe won't make much more power.
I did experiment a few years back , 2 1/2 DP , a 2 1/2 inch Magnaflow 400 catalytic converter, a 2 1/2 inch 30 inch long resonator, then a 2 3/4 inch Magnaflow muffler. A waste of time for me, but that's how we learn!