After all the years, I haven't played around much with that cmdlet...
You need to expand the property object for counters.
But, i think i'm doing something wrong with the way i've just tested it in the lab
PS C:\powershell> get-naperfdata system| Select -expandproperty counters | ? {$_.name -like "*avg_proce*"}
Name Value
---- -----
avg_processor_busy 10243596186
The ultimate goal would be to use it like the stats command
stats show -n 50 - i 1 volume:*:cifs_read_latency -
Something like that right?