I have a viper s. It took me years to find the Ohlins rear shocks in good condition. The ride is much better than the stock shocks. However I have had stock shocks revalved and had a comparable ride. Advantage of Ohlins is the easy adjustment. I would also add that currently, I am waiting on one of my front Ohlins to be repaired. No one had parts locally to fix it. The parts were ordered from Sweden. So it is going to be a bit of a wait. As the sleds get older parts are much harder to find.
I have no experience swapping suspensions. But I bet if you keep an eye out here or ebay something may come up.
Revalving stock shocks can drastically improve your ride for less work and possibly less cash. It really depends on what deals you come across. If the sled has more than 3000 miles on it, it is probably time to go through the skid and replace warn bushings as well as a rebuild on the shocks.