LeviathanUK

There won't be a difference, because you would essentially be buying the same memory.
The only difference is that they might have tested the 8x4 kit in quad channel (as opposed to two times dual channel, which you're far more likely to run).

An AIO would definitely help. Doesnt mean that the airflow inside the case would be irrelevant though as radiators have fans, GPU needs air, some air to help the RAM integrated heatsink. 
 
Just make sure the air intake sections arent blocked by anything. And ensure regular dusting of the mesh.

Yes, the P400 is a nice looking case. Only problem is the airflow on them is rather sucky. I personally have the P400S, it does the job but if you intend to go with the 8700k and overclock it, I just wouldnt be so dead set on the case. 

But for games, 1080ti is a huge jump over 1060. If you're only looking for the fps bump, gpu upgrade would impact the most.
But looking at the past timeline, the flagship always comes first, so you'll see gtx 2080 by 2nd quarter which will drive 1080ti price down.
Do you have the patience?

I wouldn't expect Volta to be bestowed upon us until march at least, maybe even as late as June, granted I prefer to avoid getting my hopes too high so they don't come crashing down... Intel's probably not releasing a new cpu architecture until late next year, but AMD is expected to release a Ryzen optimization around February, if we're lucky it'll cut down the performance gap with Intel and spur some serious competition

Well, some games really hammer the cpu and won't get to 200 fps even on a 8700k at 5ghz, others like csgo will easily go there and beyond. The difference between an Intel cpu and a Ryzen cpu comes down to whether you can actually see the difference between 90-120 fps and 120-140+ fps most of the time. But you do you, that's what building a pc is about
gta5 is a typically heavily cpu bound game, if you watch the video the gpu never reaches 100% usage because the cpu can't keep up, the 7700k at 4.5ghz isn't doing its job

wellt the 8700k can do streaming using h264 faster, fasterer, or fastest. The 7700k drops frames like they're hot. Ryzen 7 is the best cheap option for gaming and streaming, but can't push frames quite as hard as Intel cpus
nvidia works with ryzen 
nvme offers little to no performance benefit for gaming, or most tasks outside of video editing/rendering

If you're looking for streaming, then the extra core would def help.

500 watt is enough for 1 gpu. no need to upgrade psu.
 
it comes down to the budget, do you have enough money to upgrade new mobo?
if yes, then 8700k will be huge upgrade from your current system.
2 more cores than 7700k and 8 more threads than  current one.

I run a 1080Ti FTW on a 4790K @4.00GHz and the CPU usage in Wolfenstein II was around 40%. I saw less than that in every other game in my library.
 
I should clarify I'm driving a G-sync'd 1440p @ full settings on everything.

Setup is perfectly fine, upgrade what you like and dont worry about anything.