I really fail to see the point of this discussion. 10 core > 8 core. Period. That is why the 10 core exists and costs more money. Now, if you think the better performance you get with 10 core is not worth $400, that a whole other discussion. Sure, if you want to save a few bucks, this might be an upgrade many people should skip. But don't rationalize that the 8 core performs better than a 10 core.
I agree with you for many workloads but it also depends on the use case and what your workload is using at any time, obviously if you are using a highly threaded application that requires more cores you will get better performance. Most of the time for me and I suspect many others our workloads benefit from faster single threaded and lightly threaded performance.
Then we need to take at the complex nature of Intel CPUs at the moment, sure Apple make it nice and simple on their product page but how do we get that great performance in single threaded and lightly threaded workloads? Turboboost.
There are now 3 turbo modes in *some* Comet Lake CPUs along with the base rate. All-Core turbo, Turbo Boost 2.0 and Turbo Boost 3.0.
Apple doesn't list the exact Intel models they are using and I haven't had time to research it in detail yet however you will see on the Apple page the 8-core is listed as a 3.8 (base rate) boosting to 5.0 and the 10-core is listed as a 3.6 (base rate) boosting to 5.0.
So just from Apples information alone you can see you could have a workload where you are obtaining a higher base rate on the i7, depending on the length of the workload and thermals etc.
Just to add something else in to the mix I thought I saw a video where the reviewer commented that they saw the i7 go to 5.1 GHz in Intel Power Gadget which would make sense as one of the public i7 chips is a 3.8/4.7/5.0/5.1 (base/all core turbo/turbo 2.0/turbo 3.0) however their public i9 which most closely matches what Apple is using does not feature turbo 3.0. If this is the case in the lightest of single thread workloads the i7 would be fastest.
I'm just providing this information out of interest and to educate, the differences are very small but do exist!