I expect Polaris 10 to have roughly the same shader count as Hawaii (best guess: 3072). I also expect it to have a well-balanced front end, which Hawaii and Pitcairn have, but Tahiti, Tonga, and Fiji don't.
FinFET allowed Apple to boost its SoC clocks by more than 40% over the corresponding 20nm planar chip. I think a lot of Polaris 10's gains will come from increased clockspeeds - somewhere between 1200 and 1400 MHz core clock is well within the realm of possibility. Between that and the architectural improvements, it should easily be able to beat Fiji. Remember, Fiji is only about 15% faster than Hawaii at 1080p.
As for memory, I'm going to dissent from the crowd and speculate that Polaris 10 will be using a 256-bit GDDR5X memory bus. I know some people will argue that it's too early for that, but remember that Fury X hit the shelves before HBM technically even went into "mass production". And that's assuming that Samsung and Hynix won't beat Micron to the punch. A 256-bit bus with standard GDDR5 would be too narrow for a chip as powerful as this, even at 7GHz effective clock rates. GDDR5X should allow an effective clock rate of 12GHz even early on, and this would translate to 384 GB/sec of bandwidth, the same as the R9 390X. It also means lower power usage, die size savings, and fewer PCB traces needed. First-generation HBM is out of the question because 4GB is simply not acceptable outside of the low tier any more.
Hawaii has a die size of 438mm^2. Most estimates have 14LPP providing about 2.2x as many transistors per mm^2 as TSMC 28nm. That would mean that a blind die-shrink of Hawaii would be just under 200mm^2. Some additional space will be taken up by larger caches, new hardware functions, updated UVD for HEVC, and so forth, but Polaris 10 can cut the memory bus width in half by using GDDR5X, which is a substantial space savings. It can probably also drop the Double Precision support found in Hawaii; I'm assuming AMD is saving that for Vega. Given this, it's more than plausible for a chip like this to fit within 232mm^2 on 14LPP.