Note that all the configurations are otherwise identical, and for reference, I consistently used all solid-state storage, 32 gigs of RAM (at 3,200 MHz), and an Intel Core i7-8700K processor. Even the motherboard, case, and cooling were all kept the same to minimize other factors.

One other thing to note is that I did not overclock any builds, and simply benchmarked them as "out of the box" as possible.

All builds were booted up and benchmarked accordingly using the in-game 3D Mark benchmarking software. And here are the results!

CPU: i7-8700K
RAM: 32 GB @ 3,200 MHz
GPU: GeForce GTX 980 Ti (MSI Golden Edition) [x 2]

Benchmark Score: 10,188

CPU: i7-8700K
RAM: 32 GB @ 3,200 MHz
GPU: GeForce GTX 1070 (MSI) [x 2]

Benchmark Score: 10,174

CPU: i7-8700K
RAM: 32 GB @ 3,200 MHz
GPU: GeForce GTX 1070 Ti (MSI) [x 2]

Benchmark Score: 11,179

CPU: i7-8700K
RAM: 32 GB @ 3,200 MHz
GPU: GeForce GTX 1080 (MSI) [x 2]

Benchmark Score: 11,990

CPU: i7-8700K
RAM: 32 GB @ 3,200 MHz
GPU: GeForce GTX 1080 Ti (MSI) [x 2]

Benchmark Score: 14,241

CPU: i7-8700K
RAM: 32 GB @ 3,200 MHz
GPU: GeForce RTX 2080 Ti (ASUS ROG OC Edition)

Benchmark Score: 10,268

The conclusion this "experiment" demonstrates, is that the GeForce RTX 2080 Ti is a massively powerful card. It is still, unfortunately, dwarfed by the GeForce RTX 3090 (and other members of the RTX 3000 series), but based on this data alone, the RTX 2080 Ti outperformed dual GTX 980 Ti's and outperformed dual GTX 1070's when put into SLI. The other builds - the GTX 1070 Ti's, GTX 1080's, and GTX 1080 Ti's - still scored higher.

However, like I said in my disclaimer, I deliberately did not overclock any of these builds. This means that, through overclocking that I'm too scared to do, even on virtual machines, the RTX 2080 Ti could potentially outperform these other builds, assuming the other builds aren't overclocked.

In short, the GeForce RTX 2080 Ti is more powerful than two GTX 980 Ti's or two GTX 1070's in SLI.