Bill Gehrke ... The Expert on real-time testing & hardware issues for the Adobe DVA's, recommends the Samsung T3 drives via USB3 as the biggest help for laptops to get a decently fast second drive. Real-time sustained read/write speeds while PrPro is running is crucial, and other drives simply don't come near.
Yes I have even edited simple XAVC-S Sony 4K 100 Mbit/s footage on my USB 3.0 port on my PC laptop that is slightly older than your Macbook Pro with the Samsung T3 USB 3.1 portable SSD. I assume you have a Thunderbolt port on your Mac so that might also be usable but the T3 is the only solution that s well tested. I put all Project and Media on the T3 but do leave the Media Cache folders on the default OS/Applications SSD. If you have another computer some place you can plug the T3 in and edit away.
Using the T3 for almost a year now without a hitch (knock on wood). Its USB3 so I can do my AE/Premiere stuff on my MBP while travelling and then just plug it into my PC workstation (with Paragon mac drive reading software installed) and continue working on a big screen.