AMD Ryzen 9 5950X: 20.6W for a single core at 5050MHz, 49W for the whole package. (And it’s generally the package figure that you care about.)

AMD Ryzen 9 5900X: 17.9W/54W at 4875MHz.

AMD Ryzen 7 5800X: 17.3W/37W at 4825MHz.

AMD Ryzen 5 5600X: 11.8W/28W at 4650MHz (though the highest core reading is 13W, at three cores loaded).

You’re both correct: by simply restricting that power envelope by 40%, you shed a lot less multi-threaded performance than people realise, and no single-threaded performance.

Look at the 5950X figures, and you observe that at about 120W, it can run 6 cores at 4,650MHz (27,900 core–MHz), or 16 cores at 3,775MHz (60,400 core–MHz).

Expressed one way: by dropping the frequency by 20%, power per watt increased by around 2.7×.

Expressed another way: let’s skip a 65W envelope—put this particular 105W chip in a 40W envelope and you lose only 20% of your six-cores performance. Seriously. But I’m not sure what the curve would look like if you load all 16 cores at a 40W envelope, what speed they’d be going at.

(But do remember that “TDP” is a bit of a mess as a concept, and that we’re depending on non-core power consumption being generally fairly consistent regardless of load.)