The gain is very workload dependent, so there are no generally-applicable rules.

There are many applications which need synchronization between threads, so the speed of the slowest thread has a disproportionate influence on the performance.

In such applications, on X3D2 the slowest thread has a 3 times bigger cache on an X3D2 vs. X3D. That can make a lot of difference.

So there will be applications with no difference in performance, but also applications with a very large difference in performance, equal to the best performance differences shown by X3D vs. plain 9950X.

It really comes down to how much more this CPU is over the next one down if you're building a new rid for a long period of time. I'm running on a 5950X which is coming up on it's 6 years in November. I could have spend a little less on the next model down, but I expect this rig will last me for a few more years (especially with how much memory is). The per year extra expense for that CPU was almost nothing over its lifetime.

Now, would I upgrade an existing computer with a slightly slower processor with it, probably not.