I had this problem on my 1999 GS300 Sport also, never got it fixed and in the end swapped it for the 430. At first all seen well but after about 14k miles it started to vibrate at approx 60mph like the sport. Dealer balanced wheels, tracking etc, still did it, changed front wheels to rear, still did it.
Swapped the wheels and tyres for TTE 18inch, guess what? still did it. I also knew about the pretension problem of the rack (seen it on the USA sites) so I told this to my dealer who at first was interested as they said they had another of their customers with the same problem.
Then they refused to touch the rack, said they were changing the TC in this other customers car. Glyn was your car in at Coventry? But was then told that that did not fix the problem and they did not know the answer to mine.
Then in December last year i saw that the front tyres were bald on the inside edge 6mm on the rest of the tyre. So it had a four wheel alinement £150 which cured the wear problem.
It also fixed the vibration problem and its never come back.
So in my case it was not the wheels, tyres or tracking it was the camber angle :)