If you want bigger tires for the mud you should go for some 25s. I had 23s for a while but I got tired of getting stuck all the time. You won't need any mods for the 25s because the fenders have enough clearance. You should not really notice a power loss but depending on the tread of the tire, it will be a little harder to turn (but that is to be expected)
But if you are as serious as me in the mud, I went and bought a 2 inch lift kit and 27 inch dirt devils.... very rarely do i get stuck and ive never had that much power loss.... but overall if you are in some mud but you want it to feel as stock as possible I would recommend 25 inch mudlites, they should get the job done.