I've taken the vehicle on a long drive on the freeway yesterday to test the effect of the changes I've made. The conclusion is that the vibration at 104 - 118kph is significantly reduced and the vibration under load is also significantly reduced to almost non-existent.
The things is did were, in order:
1. Had both rear wheels refurbished (check for buckles, cracks and respray). They found right rear to have 3 cracks which were repaired, buckle which was repaired and the whole wheel resprayed and balanced. The left wheel was found to have numerous cracks and buckled. They refused to repair and I replaced the wheel.
The mag shop gave me a loan wheel from a Z4 which gave all sorts of errors and I just decided to park the car until i get my new wheel.
After replacing the wheel, the vibration was slightly improved, but still there and I was disappointed.
2. I then decided to do the road force balancing before looking for fault elsewhere on the drive train. The results were immediately noticed, however the Hunter machine showed a big unbalanced mass on the left rear tire which couldn't be balanced out. I had to replace the tire. I logged a claim through Goodyear's technical representative. They approved the replacement. The new tire was fitted and the improvement was immediately felt. No more heavy vibration at 104 - 118kph, no more droning at 80kph.
Yesterday's drive was smooth and enjoyable for a change. There is still some road vibration, very slightly so, which I will accept, given this vehicle's sports suspension, run-flat tires and 19" wheel size.
Hope this helps others.