way back when i first purchased my rhino the first thing i did was install that foam filter with the billet collar,,, i swear i did not notice any difference from stock. none.
1 season later i upraged the intake to the K&N highflow intake,,, ground out the header welds,,, removed spark arrestor,,, and jetted my carb. big difference from stock, you could actually feel more power.
1 season after that i took out my stock modified header/muffler and installed looney tuned duals... didnt notice any difference over the modified header/muffler.
soon after that i upgraded my motor to a 686 with a coulple extras, and i do believe that the duals work better for that motor than the modified stocker would have.
moral of the story,,, if you have a stock motor duals are a waste of $$$. if you decide to do a 686 upgrade you should see a bigger power increase with duals or ron woods set-up.