I've done a cost / benefit analysis on the 821 and 841, and they are both far better than the 741 for power usage and hash rate.
How can one do a "cost / benefit analysis" without knowing the cost? Asking for a friend.
I think he just calculated that (7.3TH/s divided by 1150 Watts) is less than (11TH/s divided by 1200 Watts). But yeah... obviously.
If anyone is willing to pay the price for the 821 (they will), then it doesn't matter what the actual ROI is... the fact that a more efficient machine is mining the same currencies immediately reduces your earnings. This is why you can't just decide to fill a warehouse with Avalon 6s to avoid buying Avalon 8s.
It's not the efficiency that matters, it's the overall hashrate being added to the pool (and the timing of it being added) that affects earnings. Network difficulty is what drives earnings, and network difficulty is a function of network hashrate.