You have experienced the exact same problem that has been nagging me for the last 2 years.
Had wheels checked, straightened, re-finished, rod force balanced multiple times. New tires, and btw same as yours: Firestone Firehawk Indy 500s. Went back multiple times to the wheel/tire shop to complain. In the end, they told me the tires (new) required a borderline amount of balancing weights, and that I should put in a claim to Firestone... which I never did since I plan on burning them up next weekend at the track
. Never again will I buy those tires.
Another issue to consider is "flat spotting". I've noticed this more on these Indy 500 tires, but I think all low profile tires flat spot to an extent. I noticed that in winter (cold) it does it more. Summer time (warm) less. I really annoys me because I spent so much time and money on my cars to get them to run well, and expect a nice smooth ride at high speed, especially from a Porsche.
The suspension bushings are known to be a potential cause of vibrations, but you would likely notice it more during braking only, especially in the front end bushings. I had that problem go away in my BMW 328. Worn bushings...