The 20% is entirely synthetic -- most real world shows they're within 10% of each other. More fun is that that 5Ghz Turbo is only possible with the beefiest of AIOs/Custom Loops.
MORE Ridiculous is a number of 9900k's have shipped with their IHS "Bowed" due to the large size of the Core Die and the IHS not being properly sized. Jayz2Cents had an i9 9900k he had to sand down in order to get it to even begin to OC and that's voiding the warranty on a supposedly "Overclockable chip"
Outside of this the minor 5% bump in speed from the i7-8700k with a 30% price jump. You'd be better off just getting the i7-8700k or waiting for the 10nm part from Intel at this point, buying a 9900k is a raw deal, is the point made by the article.
https://www.techpowerup.com/reviews/Intel/Core_i9_9900K/19.html
In relative terms of overall CPU performance, with the 9900K at 100.0 baseline, the 2700X achieves 83.2% of 9900K performance.
100 / 83.2 = 20.2% overall deficit for the 2700X
The difference in gaming is obviously lower because of GPU bottlenecks especially at 1440P, but in terms of overall compute power the 9900K is a lot more powerful than the 2700X (and 8700K).
I agree the efficiency of the 9900K goes out the window when overclocked to 5.0GHz, but so does the 2700X when pushed to 4.2GHz. Hell my 8700K @ 5.0GHz isn't very efficient either, I get 140W package power in IBT. You're better off running these high end CPUs at stock if 'efficiency' is your thing. I use 'efficiency' because even at stock these CPUs consume a LOT of power, especially the 9900K, and overclocking just adds an extra 50% power on that of that.
I disagree about the 8700K being a better deal, and this is coming as a 8700K owner. Read my post above in reply to Steve for a more detailed reason why. If I could afford $370 for a 8700K, I can afford $508 for a 9900K too. It's not like that extra $138 is going to send me broke. They are both high end CPUs, the 9900K is more expensive but if you needed the extra cores its probably worth the price difference IMO.
Seriously if you are value conscious with CPUs and price/performance is the key criteria you would get the Ryzen 2600 for $160 and call it a day. Nothing else comes close in terms of value. You don't buy the 8700K or 9900K because of their value, you buy them because they best complement a high end gaming machine that can also be good for productivity. At least thats how I see it, and that was why I got the 8700K last year and if I was building *right now* I would possibly consider the 9900K over it.
I agree the 10nm parts (and probably Zen 2) will make the 9900K redundant, at least from a price/performance POV. But those parts are at least 6 months away, so if you wanted to build a high end PC right here, right now, what do you suggest?!