Not sure if things changed between 5.0 & 5.1 but trying to set a value required 6 arguments for me. I ended up doing something similar to this:
$esxcli.storage.nmp.psp.roundrobin.deviceconfig.set(0, $null, $_.Device, 1, "iops",0)
The arguments are Bytes, Config file, Device, IOPS, Type, UseAno