Just bought and installed Vegas Pro 15. My system is pretty basic:
AMD Ryzen 1700
NVidia 980ti
16gb ram
Windows 10
Latest drivers for everything, and I've left everything stock, no overclocking or tweaking, I've found that just causes unnecessary headaches. Likewise with Vegas 15 I've tweaked nothing, left it all at stock install. Made a test clip that is 1:12 long and it has a watermark and Film Convert color grade on it. Source footage is 100mbs 4k from my Sony AX100 camera. Encode results:
Vegas 14, 4k, Main Concept: 7:14, 236mb file
Vegas 14, 1080p, Main Contept, 2:24, 116mb file
Vegas 15, 4k, Magix AVC, 6:56, 244mb file
Vegas 15, 4k, Magix AVC NVENC, 2:08, 212mb file
Vegas 15, 1080p, Magix AVC NVENC, 0:51, 109mb file
So for me on 4k footage I went from 7:14 on Vegas 14 to 2:08 on Vegas 15 with gpu encode. Everything is stock, I tweaked nothing on both Vegas or my PC, everything seems to run solid. I've very happy, no issues so far. Footage for what I film at least looks good quality with NVENC so I'm sticking with it.