Historical data for AMD's biggest / most powerful single GPU chips:
- 2013 - RX 290x - 5.6 tflops - 438mm die on 28nm
- 2015 - Fury X - 8.6 tflops (+53%) - 596mm die on 28nm
- 2017 - Vega 64 - 13.7 tflops (+59%) - 486mm die on 14 nm
Vega 64 with a die shrink got a 59% boost (Being generous - this is the liquid cooled Vega 64). Assuming a modest 55% increase in line with this, we would get 21.2 tflops for Navi's big die power hungry form.
Critically, however,
consoles do not use the biggest fattest hungriest chips. Even if Navi did yield 30 tflops at single precision, which I think would be an unprecedented jump in recent memory, the consoles would be using the smaller version of it, with much less performance. As you can see - the RX 290x in 2013 had nearly Scorpio level compute power, but the PS4 only launched with 1/3 of that.