[HummerJim]

I have an SUT with the tonneau cover. If it's parked in a heavy rain I get a shot glass or two of water in the back, but on the highway in the rain, I get very little water. I wouldn't store electronics or paper documents back there.
My two cents worth on supercharging, is that this is not a hot rod but a purpose built vehicle. The '08 and '09 have plenty of power. As for tires there was a long article in 4 wheeler magazine years ago about the months and even years of testing, Hummer did to come up with that tire and suspension geometry. If you use the search feature, you should be able to find it, but as I remember over 1000 hours of testing went into the suspension and tire selection for the H2. A lot of it done by AM General, people who really know their stuff. If you are going to offroad it, go with more aggressive tires, but having had them, I can say they will wear faster than OEM, make noise, and affect your handling.