since we have the same model i'll mention two things I did of note; one is that I used disk utility to re-partition (non-destructively) the drive into the normal partition and a 30GB "free space" partition to extend the life of the SSD. That way even if I fill it to the brim, it's not really filled up (and those extra cells can be used as spares in the long run). I also bought the "Transcend 330 lite 128GB" storage adapter that custom fits the SD card slot on the 13" 2015 macbook pro for 74$ (less than the price of the upgraded 512GB ssd, relatively), and that fits perfect esp. since I never really use the slot. I have all my music available, but not taking space on the precious SSD. It's kinda silly to have such a fast drive then fill it with data that will never change (and so you've found my OCD issue, lol)
happy computing -os
I bought a Jetdrive when I first got a 128gb rMBP, great little device and does take burden from the SSD for things that just don't benefit from speed. Still use it on my 256gb.