If you get on a untuned Duramax in the low gears your'e liable to tear something up. I have a tuner but run it in #3 position and don't hammer on my accelerator pedal. One of my Duramaxs suffered a major loss due to an Oil pump shaft shearing off. They say Duramax is at fault for not pre oiling the shaft but you can't prove it. Thats an $11K fix.
20 in tires may hit unless you have a lift kit. Besides, the larger the diameter the harder on drivelines because it takes more torque. I have 285/75r16s on Ford aluminum rims on my 06, the hit the mud flaps when turning. Besides the added expense, is it worth it?