Very interesting.
1) Open the "wall" right after the cat, it would flow exhaust directly into the 3rd chamber.
2) Open the 3rd chamber with a larger final pipe, as some have done already. (But with limited result, as we can see now why.)
The rest is not worth chasing. the bent pipe that results in the final exit pipe, starting in chamber one, has a necked down inlet with a venturi.
It has the 'db' killer just on exit of the "scrotum."
A long air hammer cutting tip might make this mod possible. Cut off the ENTIRE final exhaust tip leaving about a 3" hole. (Measure the bulged area, and cut accordingly.)
then clip off the pipe from chamber one so it's sticking OUT of the bulge you just cut. (There is a reason for it to be longer, it's going to be used to help accelerate gas scavange from the 3rd chamber.)
Now, between the pipe and the new cut opening, make your best guess where the cat is on the other side of the wall, and cut a large nearly closed C with the open part of the C facing the longer exit pipe.
BEND the C in towards the CAT. Pushing the lip of the C cut flap into, and then past the CAT if you can. My thought is the cut flap part of the wall from your C is now going to help smooth gas flow out of the cat, from chamber two into chamber 3.
If I was going to open it up, I'd cut a pipe here, cut a nice round hole in the 3rd chamber wall, and NEARLY butt the pipe up against the cat exit, flairing/venturi the area nearly butted up against the cat, while leaving the exit flat, and only go about 2 or 3 inches past the wall like the stock cross pipes, as this flows best in this design.
The result of a mod like this would be possibly VERY LOUD.
The C door? not as loud, but it's possible.
Part is going to depend on how big a C cut can be made, if it's exactly in line with gas exiting the cat etc. Any misaligned openings will disrupt flow, causing it find the path of least resistance, and that's going to flow things out the stock pipes in many cases.
Option B would involve cutting holes in the wall where the cat exits. Put as many holes into that wall as you can drill. It's going to be messy flow wise, but it's going to flow more for sure. Think shower head of exhaust gas into the 3rd chamber, and it's going to swirl around before it can be expelled either by other gas flow, or pressure. (Flow good, pressure bad.)
This is where a longer stock exit pipe up the center of the new larger final tip will help scavenge gas from chamber 3. The same idea is how media blasters work, only you are sucking out exhaust gas, not aluminum oxide or glass beads.