When SLi is supported well the 2080tis outperform a 3090 by up to 30% - witcher 3, strange brigade etc
When SLI is supported but doesnt scale well in the game then they are about on par
For every other game where SLI isnt support the 3090 trashes all over them
Thats where I sit now. I am looking at replacing mine with a single 3090 for consistency but some games like witcher 3 where SLI works perfectly, I know I will be looking at a drop in framerates. I currently get over 150fps in witcher 3 at 4k and will only be looking at 120fps with the 3090.
Good site to check framerates here
https://www.game-debate.com/hardware/index.php?gid=4072&graphics=GeForce RTX 2080 Ti SLI
Even in RDR2 the 2080ti SLI is 10fps+ faster than a 3090.
In Control both a single 3090 and two 2080tis in SLI have pretty much identical framerates around 65/66 fps
Just a shame that SLI support is so hit and miss.