Just for fun (im not suggesting this will be ZEN performance),
Take the single thread performance in CB-15 below, keep the same clocks as Kaveri and add 40%.
I agree, it doesn't look good. Kaveri overclocked has a 24% clockspeed advantage and still needs another 33% on top of that to compete. This works out to Haswell having 65% higher IPC.
Comparing stock Kaveri to the 4330, Kaveri still has a 14% clockspeed advantage, and still needs another 45%. This works out to Haswell having 66% higher IPC.
To get another point of reference, I clocked my Ivy to 4ghz (same as single-core turbo on Kaveri) and ran CB ST. My result was 142, which suggests that Ivy Bridge has ~51% better IPC than Kaveri.
A 2500K scores ~127 at stock. Stock Kaveri has an 8% clockspeed advantage, but Sandy Bridge is still 35% ahead. This works out to Sandy Bridge being 46% faster per clock.
EDIT: Let's throw in Skylake too. A stock i5 6600K scores about 164, with a 2.5% clockspeed disadvantage. That puts Skylake at 79% faster perclock than Kaveri.
~
If we trust AMD's figures that Excavator has 4-15% better IPC than Kaveri and do some basic math, optimistically (using 15% and 40%) Zen will be just shy of Haswell (within 5%). Pessimistically (using 4% and 40%), it will tie Sandy Bridge.
EDIT: Optimistically (for AMD), Skylake will still have an 11% IPC advantage. This would be game-changing, if AMD can hit this without major clockspeed losses. Pessimistically, Skylake will still have a 23% IPC advantage, which is potentially problematic. Another generation of Intel's tick-tock and you're nearing the disparity between Piledriver and Ivy Bridge, again.
So, at best, AMD is looking to match Intel's 2013 uarch in IPC.