Just doubling a 5700XT die size would put it above a 3070 and that's before clock, IPC and power efficiency improvements which RDNA2 will bring.
I think AMD will have the performance but can they match nvidia's pricing?
GPU's are not like lego, you can't just double the GPU and get double the performance. The whole architecture has to be designed to scale to a certain size. Hawaii (290X, 390X), Fiji (Fury) are prime examples of big GPU's that didn't have the architecture finesse to harness all that power. AMD are seemingly getting away from that approach.
If you just doubled the Navi10 (RX5700), you'd have a very poor performing GPU.