I am using 1080's not 1080Ti's, as I didn't want to spend too much up front right now. I have 1 more card slot to fill or replace the 970.
I wanna be frank, as far as mining is concerned, there is a world of difference between the 1080 and 1080 TI...the latter being much better suited for mining crypto coins for a profit (at this point in time). If you bought ur 1080 for gaming, and wanna use it to mine on the side, perfect..otherwise, it's the wrong card in terms of efficiency (at this time) for mining.
as always, please feel free to correct me as i'm almost sure there are other coins that i'm oblivious to.
Over here in the UK at least, you pay over 40% more for a 1080Ti compared to a "regular" 1080. There's no way the Ti hashes more than 25-30% faster than a 1080 (and even that's stretching it). So even if the Ti's faster and possibly a tad more efficient, the price difference just kills it. For me at least.
From a raw cost/hash ratio you're absolutely right. The Tis are grossly over-cost for the gains most people would see from them... BUT, there's a few secondary considerations to keep in mind if you're even contemplating 1080tis.
1) Hash density can be maximized with them. If you have a limitation of # of rigs and/or space available to mine, then per cubic inch these are the most profitable cards available.
2) With Summer upon us, and the cost of electric and the need for additional cooling rising, these cards allow for optimization of those costs as well, they're actually cheaper to operate with both of those considerations in mind, than 2 cards that produce comparable hashes (2 1070s or 480s) so there's a soft savings involved that might not be very easy to calculate there
2) Resale on these will be considerably greater and easier at the end of the mining boom (or when newer cards come out for us to start to replace our miners). 1060s and 1070s will be plentiful if/when that happens, but gamers will be QUICK to jump on a discounted 1080ti. Right now the 9xx series cards are much easier to sell (personal experience) if they're higher end, and will garner a better price overall. 1060s and 470/480s will be a dime a dozen in 6-12 months time.
3) And this is how I use them personally, the 1080ti is still the best performing gaming card available. I only run them in my primary desktops that I use for work/games. Those are idle 90%+ of the time when I'm not playing a game or using the PC for any heavy lifting, so as a single card in separate towers they provide me the best power available when I need it, and the best profitability possible for a tower that only has a single card in it.
If you're just building out a farm, and have the watts and space available then it doesn't make any sense to even look at these cards at the moment, but if you have any of the above (or I'm sure other situations I haven't considered) considerations then they might be worth it.