I believe the Mid 2015 may have different chassis than the Early 2015, I just test the NVME ssd speed on the two model, the Early 2015 has the same writing and reading speed as 2014 MacBook Pro around 1300 mb/s, however the Mid 2015 has boost up almost twice writing and reading speed around 2600 mb/s, so that's could be one of the main difference.