Just tested out decoupling my Google Voice number from Hangouts thanks to the new Voice update. Mixed results as of now. After decoupling:
• SMSs are sent and received only through the Google Voice app and voice.google.com, no longer in the Hangouts app.
• The Google Voice app and website don't support making or receiving voice calls at all.
• When I call from the Hangouts app, it still shows my Google Voice number as the caller ID.
• Receiving calls to my number rings through Hangouts *only* on my phone. (It used to also ring through Gmail on my computer.)
Tested using the Google Voice iOS app. Thanks to @eayoungs for helping me test this out!