Pascal GPUs h265 encoding is 50% or higher than h264 encoding speeds. A single GTX 1080 can nvenc h265 8k@30 10bpp. A 150 dollar GPU will outperform a 3,000 dollar CPU only server in encoding (regardless if it's h264 or 265).
The REAL blockers are licensing and, more importantly, legacy consumer hardware. Whether you want to call the latter a speed or power problem isn't the real issue, it's that not all consumers have the dedicated hardware and the older CPU then has the speed and power problems.
With GPU encoding it's important to take into account that GPU encodes are usually delivering way lower quality levels per bit, especially if you're looking for fast speeds. The difference is dramatic if you compare a Twitch stream (for example) encoded with NVENC to one encoded with x264. Of course, hardware encoders let you achieve things that you simply can't do with CPU encoding, so sometimes that quality hit is irrelevant... but if you're talking about whether to use h265 instead of h264, it's possible the bitrate and quality improvements aren't that dramatic when you're using nvenc.
Anecdotal evidence, and possibly an FFmpeg/Nvenc bug
I've found that I'm not able to encode video above the size of the VRAM on the GTX 1050 in my home server. It seems to spew out errors, despite the GPU's memory usage being roughly 60MB from ffmpeg
My GTX 1080 meanwhile seems to happily churn through anything and everything thrown at it
The REAL blockers are licensing and, more importantly, legacy consumer hardware. Whether you want to call the latter a speed or power problem isn't the real issue, it's that not all consumers have the dedicated hardware and the older CPU then has the speed and power problems.