I don't think even a single one has been shipped yet, yo it's a bit hard to say
From the state of facts it seems that the R7 370 is (more or less) a rebranded R9 270(X) which in turn is a rebranded HD 7850/7870. It might work and it might even be flashable, but I don't think anyone can confirm that yet.
Too poor at the moment to pick it up, though and experiment. Plus I'd rather wait until someone cooks up an EFI patch for it.
Also, the 285 has fewer compute, texture mapping, and shading units but the same number of render processors than a 280X, this also results in less power draw, 190W vs 250W. IMO you'd be better off getting a used 280X.
The chipset is the same as in the cards I've mentioned above. But as I've said, not a single card has been shipped yet, so no 100% confirmation. On the other hand, there's no real point to wait for the R9 370, you could just get a used R9 270X or HD 7850.
Doesn't look like this would happen, netkas and MVC have already played around with Tonga/Hawaii cards (to no avail so far). The older Tahiti EFI (used for flashing R9 280) is incompatible with the newer cards, and the Tonga EFI (from iMac) is incompatible with the old cMP.
It is closer to a R7 265 ( and older HD 7850 ) than a R9 270. ( There is a chart here http://www.anandtech.com/show/9387/amd-radeon-300-series/2). Both 'old' and 'new' are in the R7 class. Basically getting some small tweaks so that it is enough of GCN 1.1 to make OpenCL 2.0 work (shared memory aspects of memory handling subsystem ). Visual graphics wise there isn't a change. It is likely different enough so that trying to jam the unmodified previous generation's low level firmware at it is problematical.
It is has a "Pitcairn" base but it has compute/shaders turned off to drop into the cheaper R7 price points. I think Apple is going to stick closer to the R9 zone for their solutions so the "trickle down" coverage with be largely in that zone.