I think that this
IOPSMax = [math]::Round($writeStat.Maximum+$readStat.Maximum//$interval,0)
IOPSAvg = [math]::Round($writeStat.Average+$readStat.Average/$interval,0)
should be like this
IOPSMax = [math]::Round(($writeStat.Maximum+$readStat.Maximum)/$interval,0)
IOPSAvg = [math]::Round(($writeStat.Average+$readStat.Average)/$interval,0)