adobe added a feature/setting called hardware h264 decoding. release notes say it only uses the intel iris graphics, which is kinda rare to find. most intel cpu's like the i7-7700k will have the intel HD graphics instead. most people here have a cpu without any intel graphics, like an intel 6 or 8 core, so there isn't alot of feedback/info here with results of the intel graphics or iris working with that feature. this feature would only work with h264 media, so if you are using other codecs it won't help and instead you might want to focus on a more powerful cpu.
as a side note, amd zen/ryzen will be coming out early next year too. its rumored to be very competitive with intel cpu's and much cheaper, so it might be worth looking into. right now its a wild card, it might not perform well with premiere, or might have compatibility/bugs, or might be great and everyone will be using amd.
Premiere decoding really is done primarily by standard CPU instructions. So until Adobe gets around to the kind of changes you are asking for, IMHO you would be much better off with higher core count than that 4-core CPU
I agree with the others. That i7-7700K will not deliver enough of a performance increase over your current i5-2500K to justify the platform's $600+ expense (which means a new CPU, motherboard and RAM since you may not be able to reuse your existing DDR3 RAM in the new Kaby Lake build, which will not officially support DDR3 RAM with a DIMM voltage higher than 1.35V). As a matter of fact, if you're going to spend that much on a substantial CPU/motherboard/RAM rebuild, you're better off spending more money ($1,000 or more) on an HEDT (LGA 2011v3) build (a six-core, 12-thread or higher CPU, a compatible X99 motherboard and 64GB of RAM).