My 1080 Ti evga runs great on 1900+ resolution. Just changed to a 4K 3840 resolution monitor. went from 200 frames in BF4 to 60fps both in ultra settings. latest driver no changes. no more 1080 cards to buy for SLI setup. will a 2080 single card get my fps back up or will I need 2 way sli to get back up where I want to be? fram drop consistant in other games. world of tanks went from 400+ to 40 fps. any suggestions on card set up to run in ultra mode.

A single RTX 2080 is about on par or maybe a tick faster than your GTX 1080Ti. To see an appreciable gain you would need to step up to a RTX 2080 Ti. Don't waste your money on SLI, as pointed out above support for SLI is all but dead at this point. As for your performance, a single RTX 2080 Ti will be better but 4K gaming at ultra high FPS is still a ways off with current tech. But as pointed out above, there is no point in wanting more than 60fps if your display is capped at 60hz anyway. Yes I understand some games can feel a slight bit more responsive with very high FPS but at 4K we aren't quite there yet. 

HeavyHemi
As far as SLI support while folks say it is dying, it works exactly as well as it did before, games with deferred rendering will have some issues.

When the most globally popular PC games of 2019 don't support SLI, it's kind of funny that you would argue.  Like I said, official support for SLI is worse than ever.  Scaling is also usually bad on the performance per dollar basis, but it always was.  I didn't say that scaling is worse than ever.  I said support for SLI is worse than ever.  It's on a decline.  It appears to be dying.  No arguments to be made.

Google can easily answer your questions.
 
For example, League of Legends, Fortnite, PUBG.

If you are going to limit the scope of this discussion to the games mentioned by the OP in this thread, SLI support is 50%.  Kind of a silly proclamation to make, but you insisted.  World of Tanks doesn't support SLI.  BF4 does.
 
But I presume that his taste in games will change and that he will end up playing more than those 2 games.
 
It's a trend.  Looking at the most globally popular PC games of 2019 casts a wider net.  SLI support appears to be dying.
 
DX12 doesn't support SLI.  Think about it .... Newer games and newer APIs don't support SLI where older games and older APIs did.  The conclusion reveals itself.

Hemi, take it from someone who has 2 RTX 2080 Ti's yet I doesn't bother to run them in SLI because it's usually more hassle than it's worth. Sometimes you even get negative scaling in games where they will perform worse than a single card. Does this happen often? Of course not, however support for SLI is certainly waning and often times ignored all together. Take Red Dead Redemption 2 for example. A triple A game from a massive studio and SLI support was left on the cutting room floor. 
 
Does SLI still work as well as it ever did? Sure in the older games that support it, it's no different today than it was when they released. Sniper Elite 4 comes to mind and it's the one shining example of proper SLI scaling. Running on the Asura engine in DX12 Rebellion was able to work magic and it scales nearly 100% in SLI, it's fantastic. However the clear trend is developers are dropping support for SLI. Ever since Nvidia left it up to developers to implement with the launch of DX12 it was officially put on life support. Most developers just aren't going to waste the time and resources to code for something that the overwhelming population of gamers does not use. 
 
If Nvidia really wants to save SLI they need to develop support for it themselves at the driver level and have games see an SLI array as a single GPU so dev's don't have to really focus on coding for it specifically. Now as you mentioned games that used deferred rendering techniques will have inherent issues with SLI since it's really hard for 2 cards to take information from a previous frame they individually didn't render with typical AFR. Techniques like tile base rendering may help in that regard but even that still won't completely solve the deferred rendering issue as even in that mode a single card isn't rendering an entire frame themselves. 

HeavyHemi
Two games? No again, that seems silly.

lol
You insisted.  Where is your middle-ground?  Both extremes look bad for your argument.  Do you want to only hand-pick games which support SLI in order to validate your argument?
 
Edit:
25 of the most popular games of 2019 and SLI support:
 
Minecraft - No
League of Legends - No
Grand Theft Auto V - Yes
Counter-Strike: Global Offensive - No
Tom Clancy's Rainbow Six Siege - No
Overwatch - Yes
PlayerUnknown's Battlegrounds - Yes/No (debatable: official support was "added", removed, added, ... but it doesn't work correctly)
Apex Legends - No
Fortnite - No
Red Dead Redemption 2 - No
Call of Duty: Modern Warfare 2 - Debatable
Rocket League - Yes
Forza Horizon 4 - Debatable
World of Warcraft - Yes/No depending on DX version
Untitled Goose Game - Unknown
Rust - No
Portal 2 - Yes
Call of Duty: Black Ops IIII - No
Totally Accurate Battle Simulator - No
Shenmue III - No
Grand Theft Auto: San Andreas - Yes
The Sims 4 - No
Roblox - No
Destiny 2 - Yes
Team Fortress 2 - Yes
Garry's Mod - No
 
Remove games not released in 2019, add games released in 2019, as you wish.
 
... But there are SO MANY GAMES and it really depends on the games that you play or will play.  And it really depends on how much you want to try to hack in SLI support to games that don't officially support it and how much the poor scaling and flickering textures bother you after you hack "support" for SLI for that game.  All I can do is cast a net and make generalizations.  In general, SLI support is dying.
 
As a user of SLI from 2007 until 2018, I think I have an idea of what I am talking about.  Like Craptacularone, in the last years of my SLI usage, I often had SLI disabled due to the lack of support, poor gameplay consistency, or poor performance.  In 2018 I purchased only one video card -- for a reason.