So what we know is the RX 580 has 192Gb/s bandwidth and able to perform ~30mh/s
Well if we choose the best DDR3 memory with 1866Mhz (1866000000 Hz) then we can calculate 1 ddr3 memo chips's bandwidth. It's has a 64bit lane so 1866000000*64*8/1024/1024/1024=14.9 GB/s this is 1 DDr3 memo chip's bandwidth.
14.9*72=1074 Gb/s this is the total memory bandwidth with 72 ddr3 chip. So 1074/192*30=~168 Mh/s this could be the maximum eth mining performance with 72 ddr3 chips. Everything else is bullshit.
Well, you've got the right idea but the wrong numbers. Rx 580 @2Ghz has 256GB/s of bandwidth not 192 (R9 380 @1.5Ghz has 192). ddr3 is available in x8 and x16, and since x16 has twice the bandwidth, that's likely what Bitmain will use. Going with those numbers I come up with 250-300Mh/s.
Indeed, you are right with the bandwidth of RX 580, I found that 192gb/s from a shitty site, but your formula is wrong too. I made 2 mistakes, the first is the bad bandwidth, the second is that I used 64bit bitrate for 1 chip but 1 chip has just maximum 16 bit/clock , this is refers to the x16 (or x8 or x4), 64bit is the bitrate of the full DDR3 stick with many memo chips. So we have 3x32 1Gbit chips. Let me recalculate this: 1.866*16*32*3/8/256*30=~39.3 Mh/s