On my computer it’s not much. About 5-10fps on Nevada which still runs at +60fps most of the time. On the Caucasus map it doesn’t make a big difference for me. I guess there are other bottlenecks.
However, if you have an older NVIDIA GPU they might have a huge performance impact. A buddy of mine has two 980ti’s but the clouds get him down to less than 20fps in some situations. Doesn’t matter if he’s using SLI or not.
If you have 10XX series card it’s definitely worth a try though.