Since Ski-Doo and Rotax have switched to a chrome-nitrated piston ring starting with the 2005 model year, older model year engines have been able to benefit from the later year rings. The chrome-nitrated rings improve reliability and reduce the possibility of flaking on the type 593 (600 small-block) engines, including HOs and SDIs.
The ring deterioration seems to be more common with pre-2005 Summit models, as the rings start to flake after a really good heat-soak that is more common with heavy loading of the engine and transmission in deeper, wet snows. Some lightly-used trail models can still have rings in decent shape with good compression yet, but if you own a pre-2005 Ski-Doo 600 small-block twin it wouldn’t be a bad idea to replace the rings and get all of the power you can from it. The old rings aren’t even available any more, new ones are now automatically sent instead.