The new $isadmin works as expected in win7-32. Assuming that there's no effective difference between using /run's -a switch if mirc.exe itself is already running in elevated privileges, it might be reasonable to have everyone skip using the new API as superfluous.
One slight change might be to conform with other identifiers that return $true or $false, instead of the current 1 or 0. Unless there's the intent to switch over to this type of return value, like $compress does. I know there's no effective difference inside if() boolean logic.