Even quality brand tires can have issues, and yours are.
An out of round tire is a manufacturing defect that balancing might not solve.
A "good" tire tech can spot this on the balancer.

If you have 15x8's on the rear try changing front to rear and go for a short drive. (They should fit)It will tell you if it is in or around the wheel (maybe tires or an out of true wheel)If the problem is still there then look elsewhere..

goooooooooood idea ! will try it this weekend and see if i can nail the problem!

Mine vibrates a little too at 60+ - - i've noticed it depends on the road(as many of them are pretty crappy around here and are in desperate need of refinishing) there's a brand new blacktop right by my house that completely eliminates the vibration - - none till 90+ and even then its barely noticeable- - less noticeable than my mom's '04 honda. It might just be the roads you're driving on.

i've had my SC for about 3 months, and according to the records it came with, it was balanced and aligned about a month before i bought it, so i shouldn't have any vibe.... the guy that did it has done Porsches for 30+ years so i am pretty confident.

I mis spoke.. of course if a belt is broken it is a safety concern... I've not heard of it happening instantly at normal speeds... I was thinking that you would feel some vibration, getting progressivly worse, a bit before it "blew". ... and this would give you a bit of time to slow down or to a stop. But I'm sure tires have blown in a very short period of time, like seconds.. so your point is correct.