I’ve just done some testing using Blackmagic Disk Speed Test on my MacBook Pro 2012 model with macOS Sierra beta 7:
External USB (3.0) disk:
Internal 7200 HDD WD7500BPKT-00PK4T0 (SATA6 capable controller but disk negotiated SATA3 - up to 300MB/s):
Read: unable to benchmark system disk but declared at 160MB/s
Write: unable to benchmark system disk but declared at 160MB/s (?)
I have to say that building a fairly moderate Meteor app was painfully slow.
Internal OWC Mercury EXTREME Pro 6G SSD (SATA6 in CD bay):
Building/running the same Meteor app off of it becomes bearable, but…
Internal OWC Mercury EXTREME Pro 6G SSD + another one in the CD bay, both in RAID0 (stripe):
…still not beating ~2,500MB/s in latest MBPs of course, but makes building/running the same app quite comfortable
NOTE: If you go for the latest case, make sure you do regular backups because failing only one SSD will destroy all your data!