Answer

I got my hands on an extra set of OEM mufflers with about zero miles. Thought I could open them and see if they could be modified for sound (and maybe performance but I doubt it)

As I thought they are 3 chambers mufflers with a flapper. The flow is about what I expected having modified some on other cars.

Here are some pics of the inside (inlet is at the right side):






Basically, at low load, the gases go to the third chamber then to the first one thru the tiny tube on top, then out the tailpipe.
At high load, the flapper gets opened sending the gases directly to the first chamber and out the tailpipe.

Any ideas on how to modify it? While still keeping it not to loud but adding some growl?

Answer

Before opening it, I didn't thought the exit tube would be resonated (never looked inside the tailpipe before today) so my plan was to simply cut it out... But since it's resonated, I kinda cannot cut it... So I'm not sure what the best option would be...