BIG NAVI can compete against RTX 3080 but will need more power ... something near 430 watts.
AMD claimed their performance per watt increased 50% ...
This means around 15 tflops will need 215 watts. amd 30 Tflops (RTX 3080) will need 430 watts.
I think their card will land between RTX 3070 and RTX 3080 at around 350 watts and some 25 Tflops.
You can't go by FP32 compute performance as a good way of estimating gaming performance, as Nvidia made significant changes to the way FP32 works on their cards this generation. The 3080 might be able to get those Tflops in a pure FP32 compute workload, but not in games. While not exactly the same, think of it a bit like how AMD's Bulldozer CPU architecture shared a floating-point pipeline for every two integer pipelines. While Bulldozer offered competitive performance in integer workloads, its floating point performance was significantly lower.
Similarly, while the RTX 3080 might be able to get close to 30 Tflops of compute performance in FP32 workloads, in today's games it gets far lower performance than what that might otherwise imply. As a result, the 3080 only performs roughly similar to what one might expect from an 18-19 Tflop Turing card. Even at 4K, it's not much more than 30% faster than a 13.5 TFlop 2080 Ti, and is certainly not anywhere remotely close to being 120% faster, as the Tflops might suggest.
Or another way to look at it, the 3080's real-world gaming performance is about twice that of a 5700XT. So, if AMD doubles their core counts as rumored, and their performance per watt actually increases by around 50%, then they have the potential to match the 3080's performance while drawing around 300 watts. Of course, we have no idea what RDNA2's raytracing will perform like yet, so that part is still a big unknown.