I like more preload on the rear shock with a passenger on board. With a small passenger, I'll usually run six clicks out from bottomed on the preload adjustor. Rebound on the shock is also turned in quite a ways. I'm currently running it 1/2 turn from bottomed. As far as forks go, I did add 15cc of oil to each fork leg on my 2008. You probably dont need to do this on a newer machine. If you set the rear preload correctly, for weight conditions, the steering geometry stays similar to an unloaded bike, and thats good for handling.
The c14 will handle a passenger, and loaded bags for that matter, without trying hard. Many bikes lose the "fun" factor when carrying a passenger. Not this one. The only sketchy passenger I've carried outweighed me by....a lot. Lets be honest here, this lady could have beat up Rosie O'Donnell. I weight 180 and this lady was at least 80lbs over that. The bike did well, but stopping was not much fun.
A back-rest is nice for the a pssenger. They'll feel more secure and are less apt to do stupid things, like put their feet down, when they feel ok. The only other mod that helped the bike was butterfly removal and a Powercommander V. These mods gave the bike a lot more low end and make pulling away from stops very easy. You dont have to do this, but its very nice.
