David Taquet :
After I posted this thread, I actually found this video
https://www.youtube.com/watch?v=daiF6lYguN4
It describes performance on two cpus (i7 4790k and i5 750), with the same GPU. And it seems the i5 is slightly faster in most games at 4K. NOt sure what to make of it...
Considering his testing method, that's probably within the margin of error, so I wouldn't say that the overclocked i5-750 system was necessarily "faster" at 4k than the 4790k at stock clocks, it just performed very similar. Any differences that did exist might have been down to RAM timings or motherboard performance or some other factor. The takeaway from that video is that at 4k, even a 1080 Ti can't manage high enough frame rates in most modern AAA games to run into much of a CPU bottleneck on either of those systems. Either CPU should provide sufficient performance to perform its calculations in any of those games at least 60 times per second. It's when you start dealing with screens that can display higher frame rates that those differences become more notable.
For your system, the same is likely to hold true. It probably won't make much of a performance impact in the vast majority of current games whether you're using the i5-4690 or the i7-7700. As long as they can handle at least 60fps, it shouldn't matter much if one is technically a bit faster than the other. I would hardly say the i5 will struggle with a 1080 Ti, when it only needs to push 60fps. Now, if you were looking at getting a monitor with more moderate resolution and a higher refresh rate, like a 1440p 144Hz screen, then the 7700 would likely manage higher frame rates. Its per-core performance should only be around 15% faster though, so the differences probably wouldn't be huge even then.
The i7 does have hyper-threading though, which could help with performance in games that fully utilize more than 4 threads. The vast majority of games do not though, and and you can probably count those that do on one hand. That could change in the future though, and both AMD and Intel will be offering "mainstream" CPUs with at least 6 cores soon, so developers might be more likely to make greater use of them. In that case, combined with its slightly higher performance, the i7 might have more of a performance advantage in future games.
Now, it sounds like you already own both systems, and if that's the case, it would make sense to pair the i7-7700 with the 1080 Ti. It might not make much of a difference in terms of performance now, but in a year or two it could in newer, more demanding games. You did mention the i7 system having less RAM though, so you would probably want to upgrade that to 16GB at some point. Much like the advantages of having more than four threads though, having more than 8GB of RAM probably won't make a big difference in most existing games, though some are starting to hit that limit. Unlike CPUs though, upgrading RAM usually tends to be relatively cheap, particularly if the motherboard has some empty RAM slots available.
Back on the topic of the monitor, if you're not entirely set on getting a 4k screen, it might also be worth looking into 1440p 144Hz screens. Since they require the card to render fewer pixels per frame compared to 4k, the 1080 Ti should be able to run most games at max settings at around 100+ fps (And in that case, the i7-7700 might make even more sense). The 1080 Ti will also be able to maintain 60+ fps for longer in future games at max settings, where at 4k, you'll probably need to turn down the graphics options in many games being released a year or two from now to keep yourself around 60fps.