But also I understand the GPU is using a video encoding/decoding chip which will have a fixed performance whatever the model of GPU within a generation, whereas

But also I understand the GPU is using a video encoding/decoding chip which will have a fixed performance whatever the model of GPU within a generation, whereas software can fully utilise your CPU, and scale up if you run multiple streams in parallel. All of that is irrelevant for real time decoding, but for converting files, my experience if you are using hardware encoding is that hardware decoding can become the bottleneck and you may get faster performance with software decoding + hardware decoding.