For the most part!
I never updated this thread, so this reply will serve as the update. First off: Screw that corkscrew insert! I could feel the sluggishness and, according to my DIC, it cost me at least 1 MPG at 65-70 MPH. I immediately went home and cut it out, rewelded the tailpipe back on and went right back out for another test. The sluggishness was gone and I was getting 1-1.xx MPG more at 65-70 MPH.
At first, I thought my dual mufflers made barely any difference. It turns out the exhaust valve leaks a lot of air and sound. So, the straightpipe side, that runs parallel to the mufflers but is turned down in front of the axle was creating a lot of noise that reverberated under the cabin. With it plugged off (stuffed towels in it), it's quieter than when I had cats. So, 40" of muffler might be too quiet for you. I'll be merging that straightpipe side back into the pipe leaving the muffler and arching over the axle so whatever sound "leaks" through it will just join the muffled side and go out the back. I think it's gonna sound pretty wild outside and be plenty quiet inside. Of course, I'll be able to electronically switch it so that it'll be primarily flowing through the straightpipe side with a little "leaking" through the muffled side.
I have the 12259 clone, which is an 18" body with offset/center configuration coupled with a 12579 clone which is a 22" body with center/center configuration. Basically 40" of the same type of muffler with a little offset on one end. Your 12599 is a dual in/dual out configuration with internal X-pipe. It's the same 22" body size as the 12589 and 12579. I'm guessing that because it has two 3" pipes going through it instead of just one, it has a lot less packing to do as much muffling. Based on my experience, you need more muffling to make up for what it's lacking.
It's hard to say what the resonators will do. Many companies make a small muffler that basically just a much smaller version of the regular muffler and call it a "resonator". These Magnaflows (and their clones) we're discussing are all pretty much just glasspacks but with a lot more body to hold a lot more packing. It's a perforated pipe wrapped in packing and encased in an oval-shaped body. Many "resonators" are the exact same thing, but much smaller. I think an actual resonator is a more calculated design that manipulates the sound waves rather than just absorbs them. The factory resonator at the tail end of the system is an actual resonator. It's a Helmholtz chamber. But, instead of being a pipe that extends off the main pipe, it's like a pipe "rolled over onto the other pipe". It's length, volume and location in the system has been calculated to target a specific frequency.
If them Vibrants actually have some sort of a hollow chamber in them to reflect and manipulate sound waves, then they are truly resonators. What frequency they'll target is a wild guess. They may just be supplemental mufflers, though. Last I looked inside one, I saw a perforated tube going through a stainless body. No clue what was between the tube and body, though.
My thoughts on location are that they'd be more effective after the muffler. I see them more as a "clean up" device. The muffler's main job is to absorb to attenuate sound, although some reflecting/manipulating will take place since it is a chamber (just not hollow). So, whatever notes that still make it through the muffler will at least be weaker. The resonator or smaller "after-mufflers" act as a secondary "filter". I think that, due to their size, they'd be much less effective if located ahead of the muffler. It's like using a toothbrush to scrub your entire dash, then using the vacuum to try to clean the crevices versus cleaning the large area with the vacuum and the toothbrush just for the crevices. Also, temperature and air velocity play a role in the sound waves. The temperature and velocity of the exhaust air will be different after the muffler than before, so this will affect what frequencies are left for the resonator and/or supplemental mufflers to act upon, rendering them either more or less effective.
