For SLI, you need to have same model GPUs (e.g GTX 1080 Ti) and it doesn't matter which series they are (e.g MSI Lightning Z or Nvidia Founders Edition).
Though, do note that many games aren't SLI optimized and running SLI hurts the performance more than running single GPU. And for those games that are SLI optimized, getting 2x GPUs to work together is a nice headache. Also, you won't get 2x times the performance out of SLI. At best, you'll get 50% out of the 2nd GPU's performance while the power consumption and heat production is doubled for GPUs.
As far as what you can do is to upgrade your GPU. You got 2 choices here:
1. Upgrade your Pascal architecture GTX 1080 Ti to Volta architecture Titan V,
comparison:
http://gpu.userbenchmark.com/Compare/Nvidia-GTX-1080-Ti-vs-Nvidia-Titan-V/3918vsm395529
buy from Nvidia: https://www.nvidia.com/en-us/titan/titan-v/
2. Wait until Turing architecture GTX 2000 series (e.g GTX 2080) is hitting the shelves in July.
In gaming at 4K, you get on average 14% better performance with Titan V over GTX 1080 Ti. Also, you can OC the Titan V to get additional 10% - 15% performance out of it,
further reading: https://www.pcgamer.com/the-3k-titan-v-is-the-fastest-graphics-card-even-though-its-not-for-gaming/
As far as CPU upgrade goes, here you have again 2 choices:
1. Update the BIOS on your MoBo to the latest one so that you can upgrade your CPU to i7-7700K,
i7-6700K vs i7-7700K comparison:
http://cpu.userbenchmark.com/Compare/Intel-Core-i7-6700K-vs-Intel-Core-i7-7700K/3502vs3647
pcpp:
https://pcpartpicker.com/product/VKx9TW/intel-core-i7-7700k-42ghz-quad-core-processor-bx80677i77700k
2. Buy the new Z370 chipset MoBo so you can use i7-8700K,
i7-6700K vs i7-8700K comparison:
http://cpu.userbenchmark.com/Compare/Intel-Core-i7-6700K-vs-Intel-Core-i7-8700K/3502vs3937
pcpp:
https://pcpartpicker.com/list/ZrHBXH
Going with Intel's 8th gen CPU would yield better results since you'll get 2 more cores and 4 more threads over 4 core 8 threaded i7-6700K, making the total to 6 cores 12 threads.