With Ethminer 1.6 when going from the GTX 1080 Ti to RTX 2080 Ti is a 57% performance increase for mining on this hardware, but when going from the GTX 980 Ti to GTX 1080 Ti was a 76% increase. But it's also possible with forthcoming Ethminer updates we could see better performance out of these Turing GPUs. Anyhow, in terms of raw performance it's a nice upgrade if you have the hardware.
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One of the most important aspects though for cryptocurrency mining is the performance-per-dollar and that's where the RTX 2080 Ti at $1199+ doesn't fair well... It's possible the RTX 2080 (non-Ti) will deliver better in this department, but unfortunately I have no RTX 2080 for testing at this time. So the RTX 2080 Ti does deliver strong mining performance just as we saw with
the great showing in the OpenCL/CUDA benchmarks (and a follow-up yesterday of the
Folding@Homeperformance benchmarks), but due to the high cost, the RTX 2080 Ti likely won't be a big hit with miners.