To be honest it depends on how far you want to go and how you plan to use your system. Things like video editing, file compression, rendering where they can use multiple CPU cores will see the increases in performance with increased clock speed. In benchmarks i have seen going past 4.8 it may be that the differences are so small as a different area of the setup would be a bottle neck or is just the maximum performance with the components.
The price increases on the OC site would be or the following reasons 1) OC are guaranteeing that you can push this cpu to the speed you brought and be under thermal limits based on using the recommended cooling solution. plus offering a guarantee on the CPU. 2) When they buy the processors they would be playing the silicon lottery just like you would be buying retail and some chips overclock better than others. The likelihood of getting one that can be pushed to 5.2 is a lot smaller than getting a few processors that would do 4.8.
I cant find the exact video i saw with the benchmarks, but there are a few of them around this one is similar
https://www.youtube.com/watch?v=q2H73cLbyzI and he has overclocked the 8700k to 4.8, 5 and 5.1 if you skip to 3:30 to see the benchmarks he did.
My computers arriving on the 28th and i cant wait to fire it up.