Originally Posted by itsatdm
Progressive springs are a tuner's nightmare. They have to valve not only for the usual, but differences in a changing spring weight. I used my Hyperpro springs with a Bitubo insert and they were very harsh on and off road.
Ducksbane on the other hand, used them with his Xbike cartridges on his F800gs and likes them.
Both of these cartridges are different than what you are contemplating, just do not be surprised if they are problematic.
I think the reason that the progressive springs work for me is that I am using XChallenge cartridges which are much better than stock 800GS ones, but aren't nearly as good as a "proper" Marzocchi Shiver cartridge. About the only successful mod to the XC forks was progressive Hyperpro springs, so I guess this carries on to their new life in the 800.
As few people have ever done this mod and owned both bikes I feel it's safe to say that they definately work better in the 800 than they ever did in the XC!