Cam will definitely change your exhaust tone. If you're going through the effort of camming though, better be tossing a blower on top while you're at it. Like you say, I'd make sure your current setup is running like it should before you throw any more variables in the mix. Downpipe will help your car breath better, but if you're already considering camming, I would bet in a year or two you'll be wanting to upgrade to headers instead. Something to keep in mind.

As for changing the exhaust tone, swap your stock muffler for a performance one and you'll get some more sound at idle. If you're keeping your stock res, it probably won't get too droney while cruising. I'd suggest either keeping stock res or stock muffs to keep it from getting too annoyingly loud.