For RX470, a 7 GPU rig does about 156 ETH.  I have't measure the wattage but RX470 with mod ROM are very efficient.   I actually have 7 running on a 1000W Platinum PSU when I ran the test. It ranges depending on which RX480 you're running and the ROM you're using but it can be around 180s ETH and only use around 160-170W.  If the card is not setup for undervolting and are NOT using efficient ROM, the wattage can go as high as over 200W especially the Ref 480s.

I see a lot of mental gymnastics looking for something nefarious which isn't here.

If you want to attain stable 27 MH/s, do you know you can get it without flashing through Claymore miner's integrated functions? Use -cclock and -mclock, check FAQ of Claymore miner for more information.

I see this as less of a problem. Let say he has 1% of the overall network hashrate (180 khash/s), he could only send those spammy transactions in 1/100 of the blocks found. This also assumes everyone has updated to the latest code.

 The A4's can do that then owners of the A4's would be doing it now.  You can overclock them, but you don't see the extra hash on the pool side.  For example you turn up the speed to 1224mhz, the miner now says it is hashing at 281mhs(vs 275mhs in stock settings at 1200mhz), but you won't see it on the pool. 

The problem will likely be the flood of L3's coming online and the massive increase in difficulty.  Profitability is likely going to get squeezed pretty hard here. I am trying not to be negative, but I think we have already seen the precursor with the big spike recently that we thought was the A4's.  Most likely that was due to the L3's and not as much the A4's.  Just a wild guess on my part, but it certainly fits the time line.  Not sure that these miners will be profitable for very long.

Also work up to a rig that was down from a MSI RX470 being shorted.  EVGA PSU are very smart, if there's a short.  It will not power back on until you remove the power connection to that GPU.  So I have to isolate which one and in order to get the PSU to turn back on, I have to remove the MB cable and plug it back.  After that I take the shorted GPU and I bring it to my Corsair to power it up and end it's Life fast as that's when the GPU WILL sizzle and goes to smoke.  After that it's RMA time.

To calculate the hashrate for, use what you would for Bitcoin but do 2^16 instead of 2^32. Something related to 'target bits' and I haven't fully looked into it but the numbers I get back are reasonably accurate. Certain pool front-ends also use this math for their calculations.