2015 crosstrek xv exhaust build

[driftwhore]

ive got a 2015 Subaru crosstrek with just a drop in apexi filter but the major mods were that after an accident I upgraded to the drp exhaust that's a 2.5 from the factork front resonator and runs mostly straight back to a magnaflow muffler in the factory location with a turndown exit like factory. sounded very nice except when cold it would have some rasp/putter noise until it warmed up (like 5 minutes down the road).
Now as every Subaru owner I've been of the search for the boxer rumble so I purchased a set of tsudo uel headers and put them on last week and the sound is wonderful until I hit half throttle them my subbie transforms into the cheapest ghetto Honda civic. the rasp\putter is awful but I can hear the underlying rumble and its great at anything less than half throttle.
So now I'm on the search to rebuild the catback or add another resonator
current setup is
apexi drop in filter
tsudo uel headers 2.25 pipes going to a 2.5 collector (removes front cat)
factory resonator section
drp resonator back exhaust that ends in a magnaflow muffler in the factory location and exits like factory

looking to rebuild the resonator back section with a vibrant performance 1792 bottle style resonator and ended through a cherry bomb 7426cb muffler with a custom exit.

do mufflers have a specific cfm or something like that, that I should look into?