Sorry i thought it was a 850 i have the xp 550 with zilla,s 26-9-14 front 26-11-14 on the back i don,t think you will have enough power with out a clutch kit to run 28s
you can go 28's no problem. the polaris 550 can turn 30" silverbacks stock with no problems. i would look at 28" zillas. good on trails, good in mud, last a while and they are pretty light.