nobody really knows until they release it and people perform benchmarks....but its supposed to be another 5-10 percent in cpu performance and there heavily investing in their integrated gpu....and if you do plan to upgrade to a haswell cpu then you will need another motherboard because their basing their haswell chips on a 1150 socket.
It may even be ~ +17.5% faster per clock over Sandy/Ivy Bridge, if the cache is going to be redesigned then it will keep performance higher during heavier workloads and it looks like it will definitely have more (visible) registers.
Unknown. But I'm betting on less than 10% over Ivy Bridge simply because Intel needs to focus on lowering power consumption to improve battery life in convertible laptops and tablets.
Broadwell will likely be the CPU that will see a 10%+ gain over Haswell since the transistors will be shrunk down to 14nm which means Intel can increase the performance while at least maintaining the same power consumption as Haswell.
I would therefore wait for either Broadwell or Skylake (2015) if I already have an i5-2500k.