Most modern games will already do 60fps fine on an i5-2500k or even less. The i5-4690k and beyond are just more overkill in case future games suddenly become considerably more CPU-intensive or for people who do other stuff for which they can use the extra processing power.
As for how much better it will be, nobody knows. Broadwell which is supposed to be a Haswell shrink with a few tweaks actually scores worse in some benchmarks.
Most modern games will already do 60fps fine on an i5-2500k or even less. The i5-4690k and beyond are just more overkill in case future games suddenly become considerably more CPU-intensive or for people who do other stuff for which they can use the extra processing power.