No difference. The only difference is that I tend to drive it a little harder with the stronger clutch in, so that will probably decrease mileage. If you maintain your same driving techniques, you won't notice a difference.

A little. The tranny is very heavy when you are on your back trying to push it up to the engine when the car is on jackstands. It is a very involved project, so if you don't have the tools, time, and resources, don't attempt it.