Yeah, you can graft a Deore or XT GS or SS cage onto a tricolor rear derailleur without any problems. As it was the local (SE VA) fad to run that setup, I did dozens of such grafts/transplants as a shop mechanic. The geometry of the Tricolor RD kept the cage closer to the cogs, and the long cage wrapped more chain. I think 28t was about as large as we went in the back.

Pete