You can leave the clutch installed to install the new springs, its just a major pain in the butt. Truegrit and I, well mostly him, did mine on Saturday, its a real pain, but seems worth it.
gear reduction takes care of all the bogIt isnt really going to help with the bogging. Mine bogs as well with the 29.5s. What it will help though, is when you come to a stop in the mud, and try to get moving again. It allows the drivetrain to become engaged higher by a few hundred more RPMs to help get the tires moving.