Whoa, I didn't even realize that Hey Siri wasn't included in Sierra, haha. Of all my devices my Mac is probably the one I'm liable to use Hey Siri on the most, so that's too bad.
Somewhat related - when your Macs are muted and you invoke Siri, does she still make that activation dinging noise? That always bugged me to no end and I always end up activating her by mistake in the library or some other place where I'm supposed to be quiet