It's hard to imagine that Zen will have IPC any lower than Sandy Bridge. Remember, that was a 2011 design made on 32nm, and even given the long lead time of CPU architectures, AMD had to have known going in that their new design needed to be at least that good, if not better. Most likely, we'll see IPC somewhere in the Sandy Bridge to Haswell range. The official claim of 40% IPC improvements over Excavator (which equates to ~61% better than Piledriver) would also put the result slightly above SB (which I calculate as having 58% better IPC than Piledriver, based on comparing the FX-8300 to the equivalently clocked i7-2700K on five ST benchmarks on this site).
The real wild card is clock speed. Zen will be expected to make most of its money in the server market, where core counts and perf/watt are more important than raw GHz. I wouldn't be surprised to see the flagship 8-core Zen sporting a base clock speed of 3.00 GHz, with higher turbos allowed when not all cores are in use. Keep in mind that the 8-core Haswell HEDT chip (i7-5960X) also has a 3.00 GHz base clock. Some enthusiast-focused motherboards will probably be built for a thermal design limit of over 95W, which will allow overclocking, perhaps to near 4.00 GHz.
A hypothetical Zen HEDT chip with 8 cores @ 3.00 GHz (OC to ~4.00 GHz) and IPC in the SB/IB range will probably need to have a maximum price tag of $499 to be able to take sales from Intel's HEDT lineup. They are expected to be released in H2 2016, and that means they'll have to compete with Broadwell-E. Server parts will sell at a discount to Intel's parts because they will not be quite up to par with Intel's best products, but they will at least be in the same ballpark and should be able to win back lost market share at much better margins than AMD is currently getting with their absurdly bad construction core lineup.
The real wild card is clock speed. Zen will be expected to make most of its money in the server market, where core counts and perf/watt are more important than raw GHz. I wouldn't be surprised to see the flagship 8-core Zen sporting a base clock speed of 3.00 GHz, with higher turbos allowed when not all cores are in use. Keep in mind that the 8-core Haswell HEDT chip (i7-5960X) also has a 3.00 GHz base clock. Some enthusiast-focused motherboards will probably be built for a thermal design limit of over 95W, which will allow overclocking, perhaps to near 4.00 GHz.
A hypothetical Zen HEDT chip with 8 cores @ 3.00 GHz (OC to ~4.00 GHz) and IPC in the SB/IB range will probably need to have a maximum price tag of $499 to be able to take sales from Intel's HEDT lineup. They are expected to be released in H2 2016, and that means they'll have to compete with Broadwell-E. Server parts will sell at a discount to Intel's parts because they will not be quite up to par with Intel's best products, but they will at least be in the same ballpark and should be able to win back lost market share at much better margins than AMD is currently getting with their absurdly bad construction core lineup.