RDNA1 is vastly better than GCN, its not going to be anything like that uplift, AMD said 10%+ higher IPC, we know RDNA2 clocks about 350Mhz higher that RDNA1, about ~20%, so take that and add the 10 IPC (+30% Total) and then you have a formula for an RDNA1 vs RDNA2 GPU comparison, a 2560 Shader RDNA2 GPU is 30% more GPU than a 2560 Shader RDNA1 GPU. Beyond that you add the shaders in whatever the RUMOURS are and take some performance off for scaling, with that you can get a good idea what a 2560 Shader RDNA2 GPU would look like, or a 3840 Shader RDNA2 or a 5120 Shader RDNA2 GPU.
I don't know if you're just rounding your percentages... but your maths is off again

