I've suffered the steering wobble with my GS over the years and after stripping the calipers and overhauling and most recently replacing my NSF caliper I decided to experiment. I know my discs are worn but have not been able to implicate the ball joints, suspension bushes or tie rods, one thing that always seemed worn to me was the upper caliper slider pin in it's housing.
The workshop manual I've used shows a small rubber o-ring type thing that should go on to the pin, and in a kit from Brakes International (great guys...) there is a small ring that looks as if it should fit. However the pin is not undercut as is the lower one to accept the ring and I've not been able to sort out how to fit it so have always left it out. When I renewed all my pins and relevant bushings it made a noticable difference in the clunks and creaks that come from my brakes, but the caliper can still be rocked my hand and there is a gentle clunk from the upper pin area.
So......I decided to fit lower pins with new bushes into the upper as well as the lower positions on both calipers. They were a tighter fit than the lowers but still slid and removed completely the rocking by hand. A gentle road test and there is no noise whatsoever from the front brakes now on turning and braking. I jacked the car up afterwards and the wheels turned freely with no binding so I am confident that all the pins are sliding and not causing problems.
Will be interesting to see how things pan out over the next few weeks as I keep an eye on them....I am now more of the opinion that the upper pins are a cause of some of the noise we hear from our cars and that, rather than do what I've done which may be non-standard and therefore at your own risk, perhaps replace the caliper pins, bushings AND caliper mounting plate the bores of which may become worn over the years of use.