My Day 4 review: This computer is really worth the wait.
RAM and CPU Use:
My 16GB RAM upgrade means I have about 6GB of cached files for more efficient handling of the stuff I use the most and an additional 2GB of user with my typical workload. I have yet to see the CPU working beyond 6% of its capacity, with an Air Radar open in Safari (In addition to several other processes running at 2-3%).
We all know that Mac OS is particularly good at handling any amount of RAM with good user experiences, so if you chose 8GB, I believe you'll still avoid swap, but with fewer files in that easy-access cache. The difference made would probably be minor. I expect this computer to last me for years.
256GB SSD:
That "slow" 256GB SSD feels seamless. I think we are beyond the point of the speed differences affecting the workloads of most users; take those benchmarks you'll find online with a pinch of salt.
Other: The keyboard is lovely, I really like it. As is the new Touch ID sensor, it just seems more accurate from my limited use of the old one. The screen is nice and bright for outdoor use. Battery life is excellent too.