I had them on my 4 and at times it was a PITA. Certainly runway condition specific most of the times with my 4.
I tried 40 pds and almost shook me off the runway. Low 20s worked best for me but it was never perfect.
I do know on the 4 with the stock wheels and wheel pants it was never a problem.
I have a 9A now and the gear certainly is more stout then what I had on my 4. As well I now touch down 15+ mph slower so I couldn't imagine it being a problem on the 9A. At least on the one I fly.
But the 6, its similar to a 4 in many respects. But the main gear I really don't know.
There are a few posts on the forum of guys fighting the shimmey. I didn't notice if it was more specific on one model over another but I would guess, probably.
As far as spacing from the disk to the wheel, mine was good and I didnt have to change anything there. But I have heard that other guys had to add spacing.
Tim