Yes unfortunately we do not expose device level metrics through REST
Hello   I've been trying to figure out a way to get performance metrics (IOPS, Read RT, Write RT, etc) for individual volumes (devs). Pulling this through the gui is pretty painful, but I can't seem to find it a way to do it through the api either.   I'm using the PyU4V library by Paul Martin  (great btw) and also looking at the RESTclient to navigate through the objects. The particular array I'm working with is a VMAX3 running 8.4.0.4, but we have quite a few other VMAXes running different code levels too.   Any help on this is greatly appreciated!!!
Querying device performance from REST api