I just got 27" swamp witches mounted the other day and they look great. I went with 12 in the rear. I think that the 28's might rub if the bike is stock. The 27's clear but the 28's might be a little big so flaring out the rear fender might be in order, also you will probably have to do something with the front braces in the wheel well. As far as the power loss, I haven't had it out on the trail yet but I got a HMF pipe, Dyno jet and K&N at the same time and I think it has more power even with the 27's, when I hit the gas the bike really comes alive and I can't noticed any reduction in power or performance. If you just put on the tires it will definitely be sluggish.