Perfmon stats are collected at the 'frontend', meaning they dont include any of the RAID penaltys (nor the value of array caching, etc)
They are a reasonable thing to give to your array vendor to determine what you need (let the array vendor calculate RAID penalties, etc - there are array specific subleties that matter).
Also, most vendors (including us, EMC) will want 24 hrs (at least) wirth of data so we can see the peaks, etc. A simple average isn't good enough.