PainBlame

Yet in terms of price, the 5820k is better. We can all agree on this. 

Can i run lets say two sticks of 8Gb (2x8GB) and than run two sticks of 16 GB (2x16) on the same motherboard? With the same freq (3200mhz)? Giving me a total of 48GB.
 
Mobo: Gigabyte ultra extreme gaming (rev1.0)   x99
 
Current RAM 2x8gb: G.SKILL TridentZ Series 16GB (2 x 8GB) 288-Pin DDR4 SDRAM DDR4 3200 (PC4 25600) Intel Z170 Platform / Intel X99 Platform Desktop Memory Model F4-3200C16D-16GTZB

Ideally, you should still have a metal to metal contact and the paste only fills in any gaps.  If u put too much paste the cooler will be resting on a layer of paste.

CPU speed trumps all, but that doesn't mean that CPU cache overclocking isn't important as well.  As @wrathoftheturkey stated, it's recommended to keep CPU clock speed and CPU cached clock speed withing 400 MHz of one another if your CPU's cache isn't that great and won't overclock too high.
 
On x99, CPU cache directly impacts the memory bandwidth of your DDR4 quad channel or whatever you are using.
 
In an ideal overclock, you would run a 1:1 on CPU and cache clocks.  That would be a 4.7 GHz CPU and a 4.7 GHz cache overclock.  Running a 1:1 will also increase CPU operating temperatures and power draw from the CPU.  You can add 30w to 50w extra for cache overclocking on top of what your CPU is already drawing.  This can be a small or big deal depending on your cooling.
 
To answer your question about which overclock is better, only you can answer that.  Depending on the workloads that you do, you can benefit nicely from higher cache speed and therefore more memory bandwidth. 
 
I generally run mine at 4.1 to 4.2 GHz for my daily overclock, but when I am benchmarking or trying to show off in some other way, I crank it up as high as 4.8 GHz to match the CPU speed.  Even if I take the CPU higher than 4.8, I only take cache speed up to 4.8 as that's about all it can do.

I would say if you do creative work and are rendering and such I would probably go with the XP. also with 1 card you wont have any SLI issues although the stuttering in the game you noticed could have been a driver issue or a SLI profile or lack there of in witcher 3. I currently have 1080 sli and have never had any stuttering issues what so ever. I run 1440p 144 hz. hope this helps

The 5820k, with good enough cooling, can be overclocked as fast as a quad of the same generation so you don't loose out on performance there. Skylake can be slightly faster but it doesn't offset you will have 6 cores.
 
If it is worth it is up to you. Where I am the 5820k is a little more expensive than a 6700k, plus for a comparable quality motherboard the x99 will cost more. And you don't want to skimp on the ram if going x99. So, it will cost more, to get you more. For me, a 6700k has handled pretty much anything gaming I throw at it. Can't tell the difference in real world between it and the even cheaper 6600k.

Your laptop has hyperthreading then, it adds an extra thread per every core that is on the cpu, but is not as fast as a core, there is a good techquickie video on this. 
 A six core hyperthreaded cpu will have 12 threads not "processors" those are the entire cpu. 
 
There are a few games that will use up to 16 threads now, but eight threads is the max that game devs are going to code for, reason being is that any 8+ thread cpu is going to be an enthusiast cpu, and will severely limit the audience that can enjoy the game. 
 
Get a 6700k, a z170, it will last a long time, for gaming the extra cores on the 5820k and such aren't worth it.