I'll be honest, I think that's why nVidia pushed the review embargo back to the 16th. If reviews dropped on the 14th that would have given AMD firm numbers to drop an announcement/leak on 15th/16th that showed big navi beating them. However likely you think the scenario might be, pushing the embargo back to 16th shuts the door for any possible AMD marketing before release on 17th.
Edit: Obviously AMD could make a blind announcement without firm nVidia numbers - but it's more of a gamble for them.
Or they could look at the reviews on the 16th and post something on the same day?
They already know what their own GPU's can do. They only need the Nvidia info to decide *how* they want to present what they already know about their own hardware.