I just bought a 2007 Suburban to tow our toy hauler up north. I got a 4X4, with 4,10 gears, but it is only the 5.3L and we are under the gvw at about 13,900 pounds. We were going up the hill towards payson, AZ and we were floored doing only 25 mph. After that very depressing trip we quickly added a K&N cold air intake and a performance catback exhaust. We tried it again with no more luck than the first trip. Does that sound like the right kind of performance? The dealer seems to think that is sounds about right?!?! Any help from anyone would be much appreciated!