Make it much more durable and last longer between diff rebuilds. Just using the carbide diff balls really helps to make a smoother diff. Over a short time, the stock diff balls turn into egg shape then the diff feels extremely gritty. It actually destroys the diff washers/rings really fast once the diff balls get out of round.
The al diff gear just makes it last a long time so you don't have to deal with it. It also lets you get away with being harder on it. It's easier to fix a broken axle or stripped spur than it is to replace the diff gear.
IMO, I'd to all I mentioned. It really will help durability. The diff nut is nice. I stripped out/broke about 5 of those stupid mini aluminum locknuts and the plastic thing it sits in. This piece replaces the nut and the plastic thing.
It probably would be a good idea to replace the main external diff bearings on the outdrives with some nice rubber sealed ones as well. The metal shielded ones crap out pretty quickly.
Just to put things in perspective for you:
1) I'm a basher only, so I'm extremely hard on my stuff.
2) I don't like my weekend bash sessions cut short... ever!
3) I don't have a lot of extra $ laying around, so I research everything I buy (in most cases).
4) Prior to the diff upgrades I stated, I was rebuilding the diff at least twice a gallon. After the upgrades, I went 4 gallons, then replaced the rubber sealed bearings, then went another 2 gallons before I sold it. When I replaced the bearings, I inspected the diff. The RRP gear still looked like new and the washers had a mild groove wore in them, but it still felt smooth.