0.18mach is something like 19ms, that's a good number.

Of course there isn't really any magic "boarder line" number.

The issue is port noise, and port compression. Port compression makes changes to everything - freq response, impedance, excursion, group delay, etc and can do it to a significant degree if small diameter ports are used in high power applications.

Port noise is chuffing and distortion, and you want to keep that low too, also by use of big ports and flared ports, being another option.

Anyhow - 2" port will do fine for your application.