It wont make it any more smooth but it will make it last longer. Let me give a quick explination. This is in no way bashing or supporting what is available. It will just be explaining the reasoning behind running with and without a thrust bearing.
If i am running at a big event i will never use thrust bearing. I want the diff to be as free as possible with as little drag or resistance as possible. Unfortunately, this leaves me at risk of blowing my diff if i take a big hit. This is a risk i will always take because a diff with minimal drag is the most effective and makes the car handle better.
The reason for this is by using a flanged ball bearing, you are getting a thrust bearing effect without squeezing the balls between washers. The ball bearing spins about as free as an unloaded bearing even when the diff is tight. The only resistance is the balls in the spur being squeezed by the diff rings. Now, with having a diff using a new bearing for a thrust bearing will be doing the job of a thrust bearing with alot less friction. The problem is that when you take a good side shot to the rear axle, the flanged bearing can not take the side load and the inner race exploads or breaks. So in a nutshell, a diff using a flanged bearing for a thrust bearing will yield the best possible diff but will be in danger of going south on you if you take a good hit. Adding a thrust bearing will increase the life of the flanged bearing. A thrust bearing is designed to take a side load. It takes the side load off of the ball bearing and keeps the diff staying smooth for a long period of time. The down side is the added resistance of another set balls being squeezed by washers.You sacrifice the best possible diff for a longer lasting diff. The decision is up to the hobbyist as to which is more important to him or her and what makes them able to enjoy running there cars. If you have your share of wall hits, A thrust bearing is a must so you can stay sane. If you dont crash much and can benefit from a super free diff, you dont really want to run a thrust bearing.