Even if price weren't a concern, it'd still be a purchase not worth making. You aren't going to get a massive boost in Dolphin performance (Dolphin does not take advantage of hyperthreading, and it is primarily a dual-core app) outside of being able to overclock higher. As far as I know, Devil's Canyon is still based on the Haswell architecture, so it's single-threaded performance should be on par with the i5-4670K. The chief difference would be overclocking potential. Upgrading to get 400~500MHz more, regardless of price, doesn't seem especially smart when you get your desired performance already.
It'd be different if one was like me, coming from an i5-2500K, where the difference between Haswell and Sandy Bridge is close to a 30% improvement if I were to upgrade.
It'd be different if one was like me, coming from an i5-2500K, where the difference between Haswell and Sandy Bridge is close to a 30% improvement if I were to upgrade.