I would agree on waiting, while basing possible performance on amount of Cuda cores and/or supposed Teraflops performance, it'd be like 20 to 30% increase.
that is assuming that the titles don't support new raytracing or DLSS features of Turing.
The big questions without actual benchmarks are still "Will the new raytracing/AI/turing part of GPU just sit idle" while playing older games or can the GPU offload some of the work to that part making it all faster?
So.. wait, unless you have enough money to hire a guy to burn money in barrel on your backyard.