Pretty comparable, though a single 2080 Ti has the edge in general.  In straight Timespy scores, my 2080 Ti XC with hybrid and dual A12 fans, about 50C and gaming OC, gpu scores around the same as the 100th all time 1080 SLI, or well into the 1000th's against all the 2080ti.
 
https://www.3dmark.com/spy/8756819
 
https://www.3dmark.com/search#/?mode=advanced&url=/proxycon/ajax/search2/gpu/spy/P/1085/500000?minScore=0&gpuName=NVIDIA%20GeForce%20GTX%201080&gpuCount=2
 
https://www.3dmark.com/se...80%20Ti&gpuCount=1
 
With variable sli scaling, the 2080Ti has a distinct advantage.
 
The 2080S FTW3 with it's much higher power limit is worth looking at, though it obviously won't beat the 1080 SLI where there is SLI support.
 

I did exactly this.  I went from a EVGA 1080 SLI set-up in Surround to a single EVGA 2080ti XC Ultra with a single widescreen 1440p monitor. 
 
I am very pleased with the set-up.  I thought about doing SLI with the 2080ti but I really don't need it.  Performance in similar but as mentioned you won't have to deal with the SLI performance issues at times.  I also went this route to support my Valve Index.
 
I also SLI'ed for years and it seemed like it was always the thing to do.  My system seems kind naked without it, thought I kept one of my 1080's in there to support other monitors.  If I had more money I would consider it but like I said it is not a need at all. 
 
Overall no regrets with my purchase.  Very pleased with the overall performance in all games.  Can max things out.