You are missing the point. When someone says 2 cores more are not noticeable that same thing applies to every level a 10c isn't that much of an upgrade over 8c, 12c over 10, 14 over 12, 16 over 14, and so on. If adding two cores isn't helpful where does it end (like he said Single core).You cant really reduce this by some mathematical formula. I would prefer 7940x or 7960x over 7920x too, but saying its not really better than 7900x is just silly no offense
And while i guess you would certainly see a difference between 1-core and 3-core CPU, 10/12 and 16/18 are pretty similar ratios and if anything, you would see slightly bigger difference between 12 and 10, than between 18 and 16 (assuming same clocks).
There are other reasons beside little difference that makes the 7920x a bad chip. One of those is MCC die power usage even though its only 2 cores more than the 7900. That its base and all core clocks are smaller than the 7900 by such a degree it makes the extra 2 cores be not worth it (over all processing power is roughly the same). Another big decider is people typical buying trends. That would be the three major tiers. First is cheapest that fits requirements. Second is value, which chip is the best value of the product line (why back in the day I got a 7950 over a 7970, or today people would get a 1080 over a 1080ti). Third is absolute best. A 7920x doesn't fit any of those catagories. So the only thing it has going for it is possibly being the best a person can afford, which honestly isn't going to be that much, I mean how many people are going to be putting together a 3500-5k system and choose the CPU as the item with a limited price tag and even then how can they look a 7900 and its better clocks and better OC potential, and decide that the 7920x is worth spending more on. the 7920x is the worst CPU in a lineup filled with confusing and oddly positioned CPU's.