I never touch my slipper apart from rebuilds, doesn't matter if the track is grass or astro.
On my Durango it's set a touch looser then the kit setting, as I found that was too vicious for me on the bumpy grass track I first ran it on, that setting has worked fine everywhere so far. All gears bar the spur are metal.
My XB4 has the slippper set at the looser end of the revised settings they give now, but that has still munched it's way through a lot of gears in the middle of the regionals
Wasn't just me either, Rob York was so hacked off with his, he sold it the day after the series finished
The gears that come with it are plastic, I've got metal diff internals now to stop them melting. Tracks like Coastal or Stotfold it's fine, ones with big fast jumps like ARC or Boughton, or mega grip like Herts & it'll be a set a day, which is why I switched back to the Durango for the last regional at Boughton, just knew I wouldn't have to fix it that day.
Don't go too loose on the slipper though, one it makes the car slower & odd to drive. Two you can melt out the middle of the spur as the slipping gets too hot.
The new TLR 22-4 (crap name), looks good, the XX4 was mega, but at the Worlds it broke a lot & didn't like the high grip too much from the reports, need to see what it's like on astro first.