I agree with
wshowers
If you stick to higher resolutions you will do fine. If you are going for a 3090 4K is the optimal resolution. 1440p will bottleneck you and 1080p will do it even worse. The z270/170 motherboard should suffice, but I would suggest overclocking your CPU to as high as it can go while stable. You have a 1300W PSU so your system can take it.
As for the difference in performance? Yes absolutely. SLI is a dying thing (I had dual 980tis then dual Titans and even then SLI support was sketchy at best.) The RTX 3090 alone is more powerful than your 2 cards so the upgrade is simple in performance scalability. Also as you say the 24GB frame buffer on the 3090 will help with your productivity suite.
Also what speed is your RAM? If it is below 3200mhz overclock that as well until stable. These actions are to mitigate any performance issues you encounter and while they won't completely eliminate them due to the older hardware they should help you claw back some frames.
Outside of this to take full advantage you would need a faster CPU.