I've been reading this for a couple days and just wanted to throw in my experience so far with my car. Let me just say I'm done worrying about it and gonna wait until I get new tires and see if that changes anything since the Tire Shop said it's my "Chinese tires". Granted, they are cheap tires that came on the after market wheels. Here is my story...I'll keep it as short as possible.
Got after market wheels/tires at around 500 miles over a year ago. Didn't notice anything vibrating that I can remember before this but the vast majority of these miles were on a perfectly smooth interstate from my home in MO to IN where the car was bought and back to get the wheels 1 week later. Wheel shop had a relationship with the dealer and I got 1000 credit for my stock wheels after I got aftermarket wheels put on so that's why I drove back for the wheels. Then, didn't notice anything that caught my attention right away but was back on the smooth interstate.
Upon driving the car more, I noticed the steering wheel had some "shake" to it so I thought a weight fell off or something, not a big deal. Later I did take it to a professional shop tho, had the tires rotated as I'd driven on them for prob 5k with the shake since it wasn't all that bad and had them do a roadforce balance. Shake in steering wheel was then gone, but I noticed the seat was vibrating since I was watching for that after the rotation. However, I do not know if the seat was vibrating before b/c I'd just noticed it in the steering wheel and wasn't paying attention to the passenger seat.
Took back to shop and they said I had tire slightly out of round. Got new tire put on, still shook. Took back to tire shop, (2 days later) and they re-balanced on road force machine again and said my "balance had changed". I got back in car and it still had shake and they said "it's your Chinese tires". Everything else is balanced but we can't perfectly replicate every road condition so it may shake on some roads more than others. I have since rotated again from back to front and the steering wheel is still solid, but the seat still shakes. Like I said, sometimes noticibly worse than others.
That last part is actually true, so I just gave up finding out the source b/c I wanted to lower the car. It still shakes on certain roads (passenger seat vibrates pretty good) but on smooth roads, it pretty minimal but still there.
I don't know what it is, but we'll see when I get new "non-Chinese" tires if that clears it up. Good luck to all!
EDIT: Also, I've since had the car lowered, replaced the stock strut mounts w/ GT 500 mounts, struts, shocks and also had the LCA's replaced twice under warrenty before the suspension install and this did not effect my "vibration" in a good or bad way.
2012 black on black GT Premium Auto 6R80 - 400A
20 x 8.5" AR Torq Thrust M - BMR springs & Koni str.t shocks - GT500 mounts - J&M LCAs - CHE reloc brackets - J&M panhard - General GMAX AS3 - GT500 axel backs (no resonators) - GT500 spoiler - MMD tail light trim - Roush side splitters - AM upper/lower billet grill - Bama 93v2R tune - Airade CAI