How much better is the bike gonna handle if i decide to put an Ohlins rear shock on my 03 954?

 

World of difference if set up properly.

 

Your forks are much more important than your shock. If your fronts are done and setup the shock would compliment it nicely. If you don't have your front done yet, it really isn't worth it... unless you are getting it for a deal-- in which the price is still TBD, haha.