TITAN Z was supposedly going to be released on April 29th, but according to SweClockers plans have changed. The only question is, why?
NVIDIA GeForce GTX TITAN Z launch postponed because it was slower than R9 295X2?
Before we get into details, let me just make it clear, that the following clocks speeds are not final. Some etailers already listed TITAN Z with product specs, and their data seem to confirm it (check below).

As soon as I learned the clock speeds of TITAN Z, I was quite surprised. Those clock speeds would not make TITAN Z competitive against R9 295X2. We would be looking at 180MHz frequency drop compared to original 780 TI. Unless the cooling system would be efficient enough, the boost clock of 730 MHz would not make this card any faster. Leaked listing reports on TITAN Z with 6GHz memory modules. However official source GeForce.com does report on 7GHz.
At least the memory size is where TITAN Z really shines. No gaming card has ever featured 12 GB frame buffer. Of course this is the total buffer for both GPUs, so very few applications will benefit from this. Upcoming DirectX 12 will most likely provide some improvements in memory affinity, and TITAN Z is theoretically compatible with the new API (actually it’s even listed with DX12 support).
Sauce:
NVIDIA GeForce GTX TITAN Z launch postponed