So it's a better choice than the others even with 2GB? I mean I don't expect to play everything on ultra settings, I've seen the numbers I know it's a great card.
The other thing to consider is that the R9 285 is the same GPU as the R9 380 except it will be cheaper. Really the only reason to go for the R9 380 over the R9 285 is to get the 4GB version of the R9 380. The R9 285 was only available with 2GB. You can find a budget R9 285 for $154.99 (after $20 MIR).
Yes the R9 380/R9 285 is a better card with 2GB than the smaller/weaker GPU found in the 4GB R9 270X or R7 370.
Yes. even despite the less VRAM. BUT I would recommend you grab a 4GIB R9-380. simply because current games are pushing a lot of memory. 2GIBS may not cut it very soon. so if you can get a 4GIB card i would advise you get it. but to answer your question, yes it outperforms the R7-370 and the R9-270x despite it having 2GIB of VRAM. and is it worth shooting for more VRAM? well for those cards you listed it would only be worth it for the 380.
Also don't bother with the 950. yeah it's new but the R9-380 will throw the GTX 950 under the bus when it comes to performance.
one word of advice never go for a 960 with 4GB VRAM it cannot use that much memory its just like the r9 270/x with 4GB i would still go with the sapphire because those cards work well and keep it nice and cool