I can't speak to the hardware decoding/encoding support issue, but the conventional wisdom is that (Without acceleration) if you want to be able to transcode multiple streams, you want a CPU with a passmark benchmark of around 2000 for every 1080p stream you want to be able to simultaneously transcode. So for six streams you'd want something north of 12k:
https://www.cpubenchmark.net/high_end_cpus.html Most of those are super expensive Xeons or X-branded chips.
The i5-6600K clocks in south of 8k, so unless the hardware decoding/encoding a) worked with JRiver and b) gave a reliable 50%+ bump, it would not be particularly suitable for your use case.
The i7-6700k or an i7-4790K would be a safer bet (clocking in around 11k)