My estimate is 20% less than I'd expect or like, multi and single threaded. At best, Haswell E performance in 12-18 months time.
I hope and want to be pleasantly surprised but I don't expect to be.
Honestly, even with what little we know, it'd be pretty difficult to make that design only 20% faster than the disaster that is the Bulldozer family. Sure, if they were starting from a truly blank slate and had no existing IP from which to propel themselves I'd never expect Haswell-like IPC.
However, AMD could have, literally, taken the ALUs from Bulldozer, slapped on two ALUs, and split up the specialization hardware between the four total ALUs... and we'd see a 30%+ increase in integer IPC provided the front-end can keep them fed (which shouldn't be that difficult considering each ALU can do LEA, mov[x], cmp, rotates, shifts, etc.).
Zen should be able to schedule 8 instructions a cycle and execute 10 (4x ALU, 2x AGU, 4x FPU). Considering how much difficulty Bulldozer* had even managing two ops per cycle, this is an enormous shift. However, naturally, there are consequences in adding more execution units, and scaling is never what you want it to be.
The reality is that we will see the core bursting through instructions and the cache system, no matter how good, holding up the show. This is great for power efficiency, but means that the design's performance pretty much hinges on the one area AMD has never really excelled at: cache performance.
They built this in only three years, which means they used as much existing IP as they could (and this fact was repeated many times). They didn't build new multiplication or division units, new decoders, new memory controllers, new predictors, etc. They just bolted together the best of what they had with some binding logic.
What's really interesting is the division of labor in the Zen core. In some ways, Zen will actually be barely any faster than Excavator as a result of anything we know about it (still can only do one multiplication and one division at a time, for example).
http://looncraz.net/ZenAssignments.html