Thread bump.
Worth commenting on here just for anyone else in the same boat. Mine did the same - in particular between 50-70mph saw the steering wheel rapidly wobble side-to-side. Luckily, I'd recently bought a set of Turinis over my OEM alloys and even with new tyres - the wobble was still there. It clearly wasn't an alloy and tyre combination causing the issue.
Balancing was confirmed as being true on all corners and the tracking bob-on. Not long after, I had the driveshafts replaced and with everything bolted back together, the wobble was still there.
In my head, I kept thinking it was the front discs. They had been on a while and I was thinking that just one of them with a slight warp might be enough to induce a wobble. More so, the wobble increased/decreased in severity depending on the steering input. I started to think of that science lesson that you did a school - where sat down with a spinning gyroscope, you could turn direction in your seat.
i had the front discs removed to check. No cracks and the measurement reading of one of them being 'out' was so minuscule, it hardly mattered. Still, I bought a new set of discs and pads from George on here.
Left it a few weeks and finally got around to having them replaced. Took the car out and BOOM - arrow straight. The wobble went away completely and I'm back to happy driving at motorway speeds again. I just wish I'd changed them sooner!