Kind of hard to know as this card hasn't been announced. Depends on how stripped down nVidia makes it. If history is any indicator, the card new card released usually beats the card its replacing offering the performance of the tier above it on the older architecture. So it' may not surpass it, but it will likely meet it's performance.
Unless AMD's talking about Polaris is just a snow job, Polaris is more of an architectural refinement to lower power but offer the same performance. If that's true, then nVidia has no reason to push their performance too high to still remain the top dog while keeping prices in check.