Unfortunately, those "horror stories" about 16" MBP + external monitors are true. I've tested two friends machines...same "performance" as mine. Multiple threads on reddit about this as well.
Now, if you're ok with exclusive use of clamshell mode with an external monitor... combined with turning off Intel's Turbo Boost via Turbo Boost Switcher Pro (3rd party app).. in other words, sacrificing that performance you PAID for...then most people find the heat / fans behavior "workable". If someone were to ask me my opinion on what to buy, and they need mobility via a laptop...then I rec 13" mbp / i7 / 32 GB ram + an eGPU.
Then again, you're editing 8k footage?!? Sorry, but I think it's a very bad idea to consider the 16" MBP. You MUST get an eGPU !!! The 16" MBP fans will be at 4500rpm or higher all the time. The machine will have a short lifespan. So, add $700/$800 eGPU to your budget.
What am I gonna do? I ordered an eGPU set up (Razor Core X Chroma+ 5700XT)...arriving in a week. That should solve the 16" mbp heat and fans issues. Then once the new iMacs come out, I'm dumping the 16" mbp for an iMac and keeping the eGPU for a powerhouse system that is cool in temps and QUIET. And use my 2017 iPad Pro for general computing needs. I'll buy an entry level 13"/14" for mobility at some point. Never again will I buy a "fully spec'd MBP". NEVER. If someone truly needs a laptop that approaches desktop level performance... BUY A PC laptop. Go spec out the new 15"/17" Dell XPS or a Razor Blade Pro.
Is it possible once Apple moves to 10nm chips in the next 16", it could be significantly better? Sure, it's possible. Once they move to ARM chips for MBP's in 2023..that could be a game changer imo. I won't buy one, regardless. Want desktop performance? Buy a desktop! I was stupid, but I've learned my lesson. Painfully. Good luck.