As far as I know, you won’t necessarily get 100% CPU utilization. I think ffmpeg will open a child for each flavour but will not shorten the transcoding time by the number of threads.
As an example, I have a live nginx-rtmp /ffmpeg setup with 4 real time transcoding flavours and 4 simultaneous live channels. The average CPU per thread is 40% and follows the colours changes. The more contrast changes you have, the more CPU will be used.
Other factors apply. If you have a SSD storage you will get a faster conversion than if you use a normal SATA storage.
When opening different one-thread exces, I noticed that all flavours and audio were more in sync. Maybe is just a feeling but I rarely have lip-syncs and other problems that were common with Wowza, for instance.