if you are going to stay with the stock headpipe, the Rush "Big Louie" 4" with the 2.5" baffle sounds real good. It is what I put on mine before I decided to put the Jackpot 2-1-2 on it.
If you decide at a later date to change your headpipe, you can just change your baffle size to match the mods ( new baffles are just $70.00)
There are so many different choices in this and it all boils down to what sound you like, If I were you, I would go to some bike nights and rallys and listen to the different combinations. You will find most people are very willing to let you listen and give you advise.
There are no power gains to be had with just slip-ons, so base it on the sound YOU like