Over the last few years I ran 1080, 1080 Ti and 2080 Ti cards in SLI. The 2080 Ti cards were on a X299 platform with a 7960x that was clocked pretty high so the PCIe lane limitation wasn't an issue, but I think the 4% figure may be a bit high depending on your particular use case.
With the 1080 on a ultrawide monitor with ultra settings in my game of choice, the second card kept me locked at 60 FPS in every instance at 3440x1440. When I upgraded to a 3440x1440 100hz panel, they weren't as good. Then came the 1080 Ti cards. They worked well. I anticipated getting a higher refresh ultrawide, so I knew I'd have issues with the 1080 Ti cards, keeping in mind SLI scaling isn't the best and is VERY dependent on your use case. I jumped on the 2080 Ti bandwagon, which in hindsight, I shouldn't have done. I'd rather have just gotten a Titan RTX if I was going to spend that amount of money, which is almost wasteful, IMO. I've gotten away from the ultrawide monitor in favor of two 1080p 240hz 24" (Asus XG248Q) which have been surprisingly good. 95% of the time with the ultrawide I'd be playing a game in a 1080p window, and watching something on YouTube on the other side of the monitor, so for my situation, two monitors really makes more sense.
All that being said...120hz is the sweetspot. 100hz is better than 60hz, but if you can hit 120hz you'll find the action nice and smooth. 144hz isn't as big of a diff as 100 to 120, just like 1440 to 240hz isn't all that much.
Poor scaling wasn't that big of a deal when the cards were $750, but at $1,300+, its a dumb place to spend money.
With a single 2080 Ti and the ultra preset in The Division 1, TD2 blows goats but that's another thread, I average in the high 100 to low 200s, which is more than enough.