Wow, that sucks and I’m sorry to hear what is going on.  I’m at a loss.  The only thing you are running that is not on  1000’s of other cars is the Nuespeed exhaust.  I’m just wondering if it is pushing back on the cat/test pipes or something similar.
I mean it’s happened with two different motor mounts and cat/test pipes where those would be the two most likely culprits, and anything else you are running is being run by literally 1000’s of people with no know similar issues.  To West’s point, I could see how stiffening everything up could have an impact, but at the same time, I’d expect to see issues on more than one car.  I only know of a few other cars with the Nuespeed exhaust, and I doubt they have all the same mods or track the car like you do.  Either way, strange stuff.