B doesn't follow from A.
Unity probably handles worker threads very badly, this is probably the most certain statement one could make. YMMV though. I tried special K/whatever on my AMD 2700x and didn't get much of an improvement back in the day. Most people are very very bad at properly measuring performance ("most people" also includes some hardware review outfits), so anecdotal stories of getting +10-15 fps are overblown to me since a bunch of other things might be different with your tests. I had gotten some initial gains, but with repeated testing it mostly seemed to be an artifact of having improperly measure cold-start-deadfire performance to non-cold-start-deadfire performance (and also confounding factors of things happening randomly in Nekataka). The more I tested it, the less it seemed like I got any gain whatsoever. (Though now that this thread has been necro-ed, I will probably give it a shot on my newer 3700x build just to see if it helps.)
However, this has almost nothing to do with CPU overheating. Any competent PC build should be capable of dissipating the heat from 100% cpu usage, and the cpu should also be capable of thermal throttling (same goes for GPU). There's nothing special about Deadfire or Unity that would cause this. It's hardware problems, pure and simple. Saying that normal cpu usage causes cpu overheating is misinformation.
The game won't run like ass on all high end systems. It's more complicated than "higher core count = bad". In terms of bad performance, I've seen reports from people with virtually identical systems as something I use, and I don't have the same problems. Saying "it doesn't happen to me" is extremely relevant in such situations, because it rules out software, or at the very least points to an underlying issue that is nonobvious and more difficult than "use this mod and it fixes everything." I've had Deadfire running on a haswell + 1060 pc and had no stutter issues (aside from the ones that always happen when you cast mirror image), so if other people with similar or better setups (including one person who had an ostensibly identical setup) are having stutter issues it rules out (obvious) software issues because the software is literally the same within a platform (e.g. steam literally verifies with checksums).
That being said, this is not a defense of Deadfire or Unity - for one Deadfire has serious cpu+memory leak issues. I think the severity is actually a relatively newer issue than from release - I don't know when it got this bad. It was extremely noticeable to me during my Ultimate run - I had to explicitly save/quit before and after my Belranga attempt because the sheer amount of spider corpses in my fight would cripple my otherwise pretty beefy PC, and if I continued on without quitting the game after beating Belranga I'd be stuck with 10-15 fps in fights, even if I quit to menu and long after I had left the Belranga map. In addition, every time you do a save/load cycle, something isn't being cleared properly (this was extremely obvious back in the day when hsip to ship combat battles or audio effects would have weird post-load effects). On both my newer build (a watercooled setup from a month ago) and a slightly older build (2700x w/ vega 64), I get 100% gpu usage and >60 fps (hard to say by how much because my main display is an hdtv with a max of 60). However, the more I save/load, my gpu usage starts dropping and my fps also starts dropping - basically my performance is becoming increasingly CPU-bound and is throttling my PCs ability to display frames faster. It is definitely not temp throttling - especially in my watercooled setup, my CPU and GPU temps stay cool the entire time. All I have to do is exit the game and come back and everything is fine again. This doesn't happen in other Unity games I've played, there is definitely something spaggheti-code going on with Deadfire (as if patches 4.0-5.0 weren't clue enough about the spaggheti code).
edit: tl;dr - instead of doing full VO they should have hired more programmers and QA testers. OR cut some features and have their existing programmers and QA testers have better coverage on the remaining features.