From the benchmarks i've seen there is around a 20-25% in single core performance and 20% in multi core. That's a good increase and if you add on top RAM & MB upgrades then surely a nice improvement?
The difference is not stock to stock and a good number are showing the 4770K at stock or medium overclock at around 4.4GHz (average) compared to a 5GHz 7700K so maybe I should have clarified more but if you have a good clock 4770K or conversely a poor clocking 7700K then you certainly wont see that.
Some of the other benches that are with the larger 25% increase are because they used older results of the same GPU for the 4770K and then done new results for the 7700K and because of course drivers have updated then it doesn't even out. Anything more than that from memory has had a GPU change also which negates the point of comparing (but they are few and far between where someone has done those kind of things).
With how things are starting to shape up with the Ryzen series and the R5 then I really don't see the idea to move to the 7700K.