maxalge :
that website is... Not really something you want to quote
it is well known that using less slots will often result in better performance and overclocking overhead with tighter timings
You don't like real world performance comparison and prefer synthetic benchmark sites by saying UserBenchmark is a site i wouldn't want to quote?
While it is true that 4x RAM sticks put a bit higher load on memory controller, all MoBos that have 4x RAM slots are able to withstand when all the RAM slots are populated with ease. Else-ways there wouldn't be 4x RAM slots on a MoBo.
While 2x sticks can give a bit better CPU OC overhead compared to the 4x sticks, OP's build has non-K CPU in it which makes that small benefit useless.
Also, what's your source about better performance with 2x 8GB rather than 4x 4GB? I'd like to read/watch it.
As far as my source goes and if you don't take the UserBenchmark as reasonable proof, how about video comparison between 4x 4GB, 2x 8GB and 1x 16GB?
youtube:
https://www.youtube.com/watch?v=BgQEG8RC9HE
It's seen from that video that 4x 4GB, at times, has better performance (FPS wise) than 2x 8GB since having many smaller sticks gives better RAM access speed than having few bigger sticks.
@OP
Thermaltake Smart series PSU is low quality unit and i'd return it back to the store. Mainly because of the poor build quality but also because fully-wired PSU gives you a nice headache in cable management. Any fully-modular Seasonic unit in 600W range would be much better choice, e.g M12II-620 EVO, G-650, Focus+ 650 or PRIME 650 80+ Titanium,
pcpp: https://ca.pcpartpicker.com/products/compare/WrNypg,Tc3RsY,R7V48d,nn648d/
M12II EVO and G series PSUs come with 5 years of OEM warranty. (My Haswell build is also powered by M12II EVO series PSU.)
Focus+ is the newest PSU line from Seasonic and it comes with 10 years of OEM warranty.
And you know about PRIME series already since i covered that in my earlier reply.