Concerned about CPU bottlenecking, but not PCIe lane, or other causes of bottlenecking? I don’t get it; there is always a bottleneck of some kind. IMHO, building a balanced machine provides the best performance per dollar as well as the most user satisfaction.
Your second GPU will only get 8 PCIe lanes; that’s an instant 2-3% performance hit, right there.
Few games even implement SLI support, so most of the time that second GPU will be doing absolutely nothing but generating heat. For those games that do support SLI, you’ll get approximately a 30% FPS boost at the cost of 100% more money for the second card.
Also, you’re much more likely to have stuttering and other performance issues with a SLI rig.
Don’t forget that your primary GPU will be starved for cool air and it will most likely run hot, which will induce premature throttling.
Even Nvidia has admitted that SLI is a scam, so they don’t even support triple and quad SLI any longer and the performance benefits of dual SLI are dubious at best. On the plus side, two GPUs look really cool if you have a windowed case and they’ll definitely keep you warm and toasty in the winter.
So, if you already have the ultimate workstation motherboard, fitted with a pair of water cooled i7-7700k CPUs and a water cooled GTX1080ti and you still have $1k burning a hole in your pocket, by all means, install a second 1080ti. But for the rest of us, we’d all be much happier with the performance of a single, most powerful GPU within our budget.
Just a thought, but you may consider spending the money for that second GPU on water cooling. IMHO, it looks even more rad than a pair of GPUs. My i7 rig is whisper quiet and it runs at 39-40C under full load. I don’t have any throttling issues due to heat and my antique GPU still produces satisfactory frame rates, because it runs as cool as a cucumber.
Here’s an interesting thread.