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Most vibrations, especially speed sensitive ones, are caused by out of round or out of balance rotational assemblies. If those check out, I would search for play in the suspension and steering systems. Since it sounds like you've done the road force balance, ask if they have tried rotating the tire on the wheel to obtain a lower road force reading. This can help. Also, have your tech check the wheels for trueness and runout. As to the suspension, the ball joints in the lower control arms wear and get sloppy. Unfortunately, you must replace the whole arm. Wheel bearings wear, but will typically manifest itself as noise long before causing vibration. Tie rods ends (both inner and outer) can get sloppy and this can cause play in the steering.
Also, make sure you check the rear wheels for trueness and balance and not just the fronts as believe it or not, problems here can manifest themselves up front too. For what it's worth, I know of one member who solved his vibration issue by swapping the front motor mount. A simple test for this would be to at a constant road speed (preferably down a hill) shift the car through different gears to obtain different RPMs, and try simply putting the car in neutral.
Any higher performance car with lower profile tires, especially on larger diameter wheels, becomes VERY sensitive to wheel/tire issues. The entire rotational mass must spin as a balanced and true unit, and this includes the brake rotors themselves.
Patrick
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