OK. here's the deal. From some of the information i've gathered, on an OBD II 5spd (in case it matters), 80mph should be ~3400 RPM's (thanks mate  ). By having someone pace me (speed sensor KIA), 80 for me is much closer to 4K.
Now, I have some small backfiring when I let off the accelerator (more than likely the lack of a cat), and you can feel the feedback and kinda "jerk" the car down the road by tapping the accelerator. We've all seen those commercials were the student driver is giving the instructor whiplash. Not THAT bad but it reminds you of it. lol.

My question(s):

1: Is there another version of the A578 MTX 5spd?
2: If so, how do i tell the CORRECT version from the incorrect version?
3: If not,
   3a: Do I have a possible "messed up" A578?
   3b: Is there another tranny that LOOKS and BOLTS UP just like the A578 that would work?
   3c: Could it be in some PCM controlled part?
   3d: Could my speed sensor (KIA) be the culprit?

I'm basically gathering info right now sooooooo............ if you actually KNOW anything about this one, my ears (eyes for the SA's) are open.

And yes, it HAS been replaced (stock one got a hole blown through it from the throwout bearing doing just that). The replacement supposedly came from an Avenger.

Our tranny for the five speed is the NV-T350.  I think the newer neons have the A578.  But I'm not positive.

VERY soon on the list. I kinda forgot the part number though. what was that Magnaflow number Nykon?