I am trying to determine if worth while to do the step up program from 1070 ftw. I think that I would have to pay $100-110 for the upgrade. I am pulling around 440 on ZEC with the 1070's. I don't think that the 1080 ti will be close to the 1080. It is more in line with the titan xp which gets 740 on ZEC.
In ROI without power in consideration the 1070 is faster than the 1080. The 1070 is also more electric efficient per hashes (regarding ZEC)
DCR and LBRY on the other hand you see a greater speed boost on the 1080 vs. the 1070 and going for a 1080 is a great idea for those.
I have 2 GTX 1070's and I plan to step one up to either a 1080 regular or Ti depending on how money is when I am out of time and what new ti editions might be added to step up (founders plus cost of aftermarket cooling is too much unless you care about density). Calculating cost of card to hash rates the 1070 is the Equihash king in both ROI and efficiency. The 1080 ti is the king for DCR and LBRY. 1080 is in the middle