In the past you would add RAM for speed or if running a SQL server but today you get a lot of your speed from SSD. http://www.tomshardware.com/answers/id-2947712/sata-ssd.html tells me it's nice but no way you are going to get a big payback.
I'd just look again at the 16GB models. Here my son went with a rather nice ROG with 16GB DDR4 and we cloned it to SSD.
My requirements leads me to a unicorn hunt so I guess it's best to ask guys who know
No requirements regarding CPU, GPU, installed memory, hdd, ssd, battery, connectivity, brand
1) Smaller than 14" unless they are lighter than 2kg
2) Has SATA because I need BIG storage
3) Has M2 because I want it fast
4) 2xSODIMMs because I want 2x16GB
5) Haswell/SkyLake because they can handle 16GB on each SODIMMs
Thank you very much