just my opinion and experience... I have a "12" sportsman 500, I put a wes 2x seat on the back of it with the foot pegs from theatvsuperstore.com (great site) anyway, I put the second seat on to bring my son along, he's only 60lb and I'm 200lbs. I'm very cautious when he's on the back with me but never do I feel it's unsafe for him. however so, my point is there are time that my GF will get on and ride instead of driving her RZR (no doubt the RZR has some weak springs, we also put on the racertech 2" lift and will be puting on HD springs... doesn't handle two grown men riding it, guess that's why we bought it for the ladies) back to the subject, my GF and I are under around 350lbs, believe polaris state the payload capacity is 450lbs w/180lb rider. 180 on the rear rack and 90 on the front. the 500 has all the power you need but it's really not meant to push around 400 plus pounds all the time. if you are going to use a 500 as a 2up vehicle, then my suggestion is to add the HD springs but also put in a better clutch kit and keep it maintained more often then recommended I think you'll wear it out fast and the money you didn't spend for the 2x you'll be spending on fixing it, bushings, bearings, brakes, etc... As far as being stable the bike is a pretty stable vehicle and if you roll it or bring the front end over you are doing something you shouldn't be doing in the first place. As always drive safe specially when your cargo is priceless!
by they way, any one get the new fender flares from Thor? how are they, do you like them? they look awesome...