@AeroF16
This might sound crazy in the end (Skip there if the story bores you), but I had the exact same issues as you had. This all started when I got rid of the stock tires and ran Nitto 555r's. Before the Nitto's car rode smooth as glass. I did all the same painstaking tasks you described through discount tire. Road Force balance, swapping tire position, etc etc. Nothing seemed too matter. My vibration was in the steering wheel as well in my lower back. Drove me crazy.
So Discount tire gave me a partial credit on the tires after months of troubleshooting. I then went with PK Design HR5's and Continental Extreme's for tires. Guess what...still had the exact same freaking problem. I was seriously going to pull my hair out. I did all the same things as before road force balance, swap tire position, alignment, Lug torque...blah blah blah...no help. I was beyond frustrated and honestly contemplated selling the car.
So I already had Eibach Springs on the way so I figured can't hurt at this point. Lowered the car got another alignment and guess what? Still had the same f@cking issue!
So I slept on it a few nights. I had already picked everyone's brain possible, checked all forums and other sources for help...No answers...No Help...same crap! Arg!
So I thought to myself, what haven't I changed at this point? What has always been the same since I first encountered this issue?
Hmm, well stock when I bought the car with Perelli tires the air pressure was always high...38-40ish PSI. With the Nitto's I was advised run 32psi. Better grip, better launch, etc. etc. Sure why not.
Since then I had always ran 32psi faithfully. I thought, could it be that stupid? No way right? So that night went into the garage and pumped the Conti's all to 36 PSI and I have never had the issue since. Smooth as glass. Typically running at about 38-40ish PSI once warmed up.
Anyway, can't hurt to try dude...Air is free and it was night and day for my Cat...I still don't get it...but it is what it is.