I had a similar problem, wheel wobble over 40mph, had them re-balanced, wobble over 60mph, took them to another tyre dealer and re-balanced again, all out as the previous tyre dealer's equipment was out of calibration. Do your spigot rings fit nice and tight?...again, I went through 2 sets off ebay then had a set specially made for my car.
I was also told that "Avon" tyres distort slightly at higher speed. I have these on the rears and I can still feel a vibration through my seat @ 80mph, only very slightly though. I can live with that.
I know how you feel though, spending loads of money on wheels only to make the car worse. Makes you wish you hadn't bothered.