With those improved SSD speeds, is there a noticeable boost in boot up time? Can you compare both computers times in boot up?
Short of transferring a 25 gb file to a multi-ssd/hdd RAID array connected to a Thunderbolt 2 enclosure(no single SATA SSD would be fast enough to go over 500mb/sec), you'd be hard pressed to tell the difference in speed. I went from a 450mb/s SSD on my 2013 rMBP to a 850mb/sec SSD (I bought a 1TB PCIe SSD on ebay and swapped it in.) and can hardly tell a difference.