if ROI (profit is also made) is achieved and you are about to reach the maximum electrical limit (with allowance) try adding the top of the line GPUs who have a very high hashrate and very efficient..one by one in the process upgrading your gear.
mining area (space) and efficiency --> put this into the equation
my mining area is already getting filled and my electrical Amperage is about to reach my target, that's why I'm looking at 1080 ti (waiting for aftermarket coolers by different brands). also waiting for vega release..
1080 ti aren't good at mining most coin. IMO stick with the 1070, or wait for the RX 5xx and VEGA.
ZEC is running FINE on some of my HD 7750 AMD cards that have 1 GB RAM.
It's ETH that needs at least a 3GB card to work now (and probably ETC fairly soon).
I don't see 2 rigs of 1070 achieving $2k/month right now (even if they are 6card rigs).
I've GOT 12 1070 cards mining at the moment, and even watching for most profitable coin they're only pulling in $1100-$1200 total - and the "hold" option I can't do as I need the money to pay bills, even when it has been a bonus to income (right now it's not a short-term additional income option, as the big rise in ETH/ZEC/XMR/DASH/etc has stalled out).
On the plus side, the 1070 also eats about HALF the power of my R9 290 cards while hashing even ETH at almost identical rate (the 290 is a beast on ETH), and its got some options for long-term profitability that do not exist for the R9 290 (or any other AMD card currently available).
It might be worth looking at the 1080, but it's profitability on almost anything is going to be about the same as the 1070 while the cost is still noticeably higher.
Depends on what you plan to do with the rig(s) and/or card(s) once the current surge in profitable GPU mining winds down.