An old couple of E2712 has 12 synchronous mirror pairs.
I try to add 7 more but it fails after the 4th : I got the message "The maximum number of mirrored pairs have been created on this storage array. Before creating a new mirror relationship you must remove one of you existing ..."
As you stated, the limit for number of mirrored pairs is 16 for synchronous mirroring. The limit does not change with the newer firmware for this platform. For the 27xx platform, asynchronous mirroring has a limit of 32 mirrored pairs.
The limit is set within our feature bundle, and the limit is not keyable using a feature bundle key. This means the limit cannot be changed for this platform to a higher number.
Some of our platforms and specially the newer ones can hold significantly more mirror pairs ( 128 mirror pairs).