No offence whatsoever, but the "narrative" in your post is a little bit weird considering that 3090 is on average (even according to Nvidia themselves) about 10% faster than a 3080.. But yes in some cases of course the difference will vary and perhaps HZD is one of them.
If you’re even considering buying a 3090 then clearly money isn’t an issue for you.
So, money aside, if you’re gaming at 4K with an RTX 2080 TI then those problematic games in which you’re down around 40-45 frames per second can be ‘fixed’ with a 3080 - but barely. Based purely on the benchmarks a 3090, however, will really put you over that hump.
I’m talking solely about problematic games at 4K here. When you’re struggling to reach that all-important 60 FPS mark then 7-10 frames is huge.