Again: power consumption is already taken into consideration by whattomine in the profit I wrote down. Except if whattomine is wrong, there's nothing to comment there. As for the space, I mentioned it as well but I'm going to start small with a few cards. Maybe in the future I'll replace 6 RX 580 by 10 GTX 1180 but for now, space doesn't matter.
You need to understand that making calculations on whattomine right now is pointless right? I mean, eth was pumped and the difficulty will catch up in few days to weeks, which means profit will be at least 40% less than the calculation says, a gtx 1080 ti will always make more profit than 2 rx 580 and use less electricity and take less space and management is a lot easier and also is a high end gpu which will last longer. I mean, even 3 rx 580 would not win me over a single gtx 1080 ti but that is me and the people saying cryptonight, there is vega for that and nothing can compete with it on that. So I would leave cryptonight to vega. If i had a a rx 580 right now i would go for eth, not cryptonight.
Also pay attention that as eth was pumped all alts are overpriced until the difficulty catches up as eth is the command in chief of alts.