upgrading drive of 2015 macbook air, such thing as too fast? I was considering upgrading the drive of a macbook air that i got and was amazed at the difference between some of the NVMe PCIe M.2 drives at around 3,938 MB per second and other SATA drives (which I am not wholly sure can be used anyway, even with adapters?) that are more in the 700-800s MB p/s.

Would the 2015 macbook air be a bottle neck thus making such a
    blazing fast NVMe M.2 drive a bit moot?
No - storage is the one bottleneck here. 


the CPU is always magnitude of times faster than storage and the bus speed between CPu and storage is also faster than both specs. 


Go for the speed if you are sure the drivers and connectors are compatible.