The RX 480 is a tossup with the GTX 1070 on memory access - which is why they're comparable at best in performance on algorythms like the ones ZEC and ETH use despite the GTX 1070 cost being almost twice as much.

 On most compute-bound stuff like Folding@Home, the GTX 1070 blows the RX 480 completely out of the water on raw performance, and is slightly superior on a PPD/$ basis while being much superior on a PPD/watt basis.

 The ONLY compute-bound project I am aware of that the RX 480 is competative with the GTX 1070 on raw performance is the Distributed.Net RC5-72 project - but on that project the R9 290 (for an example I've specifically worked with) blows BOTH of the other cards mentioned completely out of the water on raw keys/sec rate and on keys/sec per $, and is very competitive on a keys/sec per watt basis despite being 3 generations (appx) older.
 The HD 7870 at FOUR generations older and down a few steps on the "high end" level from the R9 290 can almost match the raw performance of the RX480 and GTX 1070 while arguing very well on keys/sec per watt and blowing both away VERY hard on current pricing.

 RC5-72 however only uses a very small part of the GPU specifically including ZERO floating point, and is small enough to fit in CACHE ram when it's run on anything semi-recent of a CPU, and does very little memory access at all. The closest thing in Cryptocoin mining to it is SHA256 (Bitcoin), and even that's not a good comparison 'cause SHA256 uses more memory.


 Nvidia gets the "scraps" on mining because most mining algorythms don't use most of the parts of a NVidia card that makes it competative with AMD cards on general or compute-bound usage at a given price point, and as a result few folks use NVidia cards to mine on which makes them a much lower priority for development.


 It's not "lack of development" ALONE that keeps Nvidia uncompetative on a hash/$ basis for ETH and ZEC (and derivatives using the same algorythms).
 It's the inherent design of the ALGORYTHMS that keep NVidia uncompetative on a hash/$ basis coupled with the higher PRICE of their cards that have competative memory access even when development IS mature.