Retina adds a small increase in GPU load, negligible added CPU load.
I'm driving 2x 4K BenQ displays with a Radeon HD 7970. GPU fans ramp up during screen savers, games, a few other instances. An RX 580 will handle a single Retina 4K display far easier than my old card.
So you might hear more fan noise (but probably won't outside of games). No impact on performance otherwise.