On the SSD upgrade, I just sold a late 2013 rMBP with a 2-channel PCIe SSD and replaced it with the current/latest rMBP with a 4-channel PCIe SSD. (The new machine also upgraded the cpu from 2.4gHz to 2.9gHz, a 20% bump, and the RAM is faster as well). I do a lot of photo editing work, and work with some pretty large data files. I can't see the difference in practical use. If you ran benchmarks, I'm sure it's there, but it's in the category of measurable but not meaningful.