1. I am not using one, but I have been told it is one of the better upgrades.
2. I am running 15/46 currently with an IX8 and feel like I am right with everyone else. 16/46 got too hot for my comfort (~170*motor temp after 10 min))
3. I am still new to offroad, but shocks and springs seem to be where the majority of the tuning is done. Personally, I would do those first, so that all of your setup changes are consistent moving forward.
4. Cool upgrades are the carbon fiber battery straps available from a user here. I also did the new sway bar holders and through other parts cars ended up with the light weight wheel nuts and hard shock caps. I see a lot of people running the metal front tower for durability and keeping the carbon rear. Bent aluminum will finish a race, broke carbon won't. I also broke my first part, which was the front chassis support brace (not the rear that everyone talks about).

Weekend was interesting. Had a bigger 2-day race and had the BE and my old converted nitro chassis out. I ran the BE all day Saturday, running respectable times and fairly consistent laps. Track conditions deteriorated over the day a bit, and while the car was a little loose in the back, was still feeling pretty good with it. After qual's were over, brought out the 1.0 conversion car. The thing felt great. Was very consistent, and could get much more aggressive with it and still keep it under control. It wasn't really faster, but much more consistent on the fast side of my laps. Sunday arrived with triple A mains. I ran the BE in A1 and A2 with a 5th and 7th place finish, and then decided to bring out the 1.0. A3 started rough, and was shuffled to the back. Clocking lap 1 in 12th place, the car felt good again, with some rear end wiggle, but more control and confidence then the BE. Worked my way up to a 5th place finish by the end.I could turn a faster fast lap with the BE, but could turn more consistent laps with the 1.0. Both with 1900 T8 motors, same gearing, IX8 in the BE and a Hobbywing 150a in the 1.0.