please stop thinking i don't think h265 has trouble at higher bitrates compared to h264. this is heavily dependend on the encoder used. and we are talking about hardware encoders, not software encoders that have been optimized for 10 years.
i highly doubt the NX1/NX500 would have better quality with h264 at the same bitrate. and furthermore if there's no h264 encoding chip on the board there's no chance to do it anyways.
at 4k resolution no one should be using h264, IMHO
(this was a post based on personal experience and my own opinion)