The thing is, you should have bought Bitcoins directly. I mean think of it this way. You bought miners to get the same amount of BTC you'd get if you had just simply spent the money on buying them.
the difference is , after you reach roi, the miners will generate more bitcoin, in comparison to simply buy them, where unless you invest again you won't have more bitcoin then your initial investment
and eventually something like an S7 is bound to reach roi, with its good efficiency, assuming you have a good electricity too