Try something like this
$esx=Get-VMHostMyEsx
$stat="storageAdapter.numberReadAveraged.average","storageAdapter.numberWriteAveraged.average"
$start= (Get-Date).AddMinutes(-5)
Get-Stat-Entity$esx-Stat$stat-Start$start|
Group-Object-PropertyInstance,Timestamp|%{
$_|Select@{N="Timestamp";E={$_.Values[1]}},@{N="Instance";E={$_.Values[0]}},
@{N="Read IOPS";E={$_.Group|where {$_.MetricId-eq"storageAdapter.numberReadAveraged.average"} |%{$_.Value/$_.IntervalSecs}}},
@{N="Write IOPS";E={$_.Group|where {$_.MetricId-eq"storageAdapter.numberWriteAveraged.average"} |%{$_.Value/$_.IntervalSecs}}}
}
It uses more or less the same concept as I used in my Get the maximum IOPS post.