My SLX30+ from 2007 originally had the metal clamps; then the later ones and now the latest. Daily use as an SLX20 and still going strong.
My SLX22 (2009) has also been upgraded and still going strong.
The latest clamps by-product is less repainting of "stop points" so only once a quarter instead of once a month.
And my Extreme 47 has a new set of clamps waiting for when it needs them.
Actually the new (6 month old) Gardiner brushes on them has sped us up too replacing the older white brushes and the red cill brush.
I keep promising myself a pair of new SLX22's but just don't need them as the upgraded clamps have extended their life yet again.
Nothing out there to touch the Gardiner carbon fibre-stuff especially when you factor in longevity and customer service.