Hello -
Received answer from MS. Its not recommended to use SIREN and just because the highest MOS score is higher doesn't mean its a better codec.
WMI Settings (wbemtest.exe):
Namespace: root\cimv2
Class: MSFT_SIPMediationServerConfigSettings
Property: DefaultCodec
Possible Values: RTA_NB (Default), SIREN
Still may test this out to see if there is a difference.
Thanks,