But, when you already have an SSD, the next best upgrade is a bigger SSD. It improves both performance and reliability. I've said it many times before: Adding an SSD to your computer is the single best upgrade you can perform for the money. It was not quite as fast as OWC said it would be but it was certainly respectable. I did a read/write speed test on my factory installed SSD once I'd turned it into an external drive using the OWC Envoy Pro enclosure and it had a read/write speed of 304MBps and 253MBps, respectively. Once you've installed the OWC SSD, you can use your Mac's original flash drive in the enclosure as a fast external USB drive with transfer speeds up to 355MBps. The kit is actually a pretty good deal because just to purchase the Envoy Pro SSD enclosure would cost you $79. You can also purchase a complete upgrade kit, which includes the OWC SSD, screw drivers, a USB standard A to micro B cable and an external PCIe SSD enclosure with USB 3.0 port for $399 (480GB model) or $649 (1TB model). The new OWC Aura PCIe SSDs for MacBook Pro Retina laptops come in 480GB ($348) and 1TB capacities ($598). What's incluced in the OWC Aura PCIe SSD upgrade kit. Note: A 1TB SSD is now a $500 option for the current top-end MacBook Pro ( vendor price). MacBook Pro for $2,500, the largest SSD you could get until recently was 512GB. Heck, even if you splurged and purchased the highest-end 15-in.

So, it's reasonable to assume you might have skimped a bit on the internal storage, which ranges from 128GB to 512GB. If you purchased a MacBook Pro Retina, you already shelled out at least $1,300 for the 13-in.

OWC has also offered PCIe SSD upgrades for MacBook Pro Retina laptops from 2012 through early 2013.

2015 macbook pro 13 ssd upgrade