Yeah, honestly it's not worth paying up for the higher boost. 4.3 GHz vs 4.7 GHz looks like a big difference. But the reality is once you're running a multithreaded task it drops down to around the same speed anyway. So, when playing a game it's going to be more like 4.3 GHz vs 4.2 GHz (or maybe 4.1 GHz at worst). The lower binned parts don't have the high single core boost, but their multicore frequency doesn't drop down as far proportionally.
To be completely honest, if you're running tasks that rely heavily on the absolute max single core performance then you should be buying Intel anyway. If single core performance is literally all you need then even a 9600K would get you there, just clock it up to 5.3 GHz.
I don't know what this is going to be priced at though. I guess probably $450? Otherwise it's getting close to the 3800X. More importantly, maybe it will actually be readily available whereas the 3900X would not be. This also doesn't bode well for the 3950X though.