You forgot to say very small bonus
Q6600@3ghz
Source Blu-Ray AVC -> Level 4.0 CRF22 (1920x800)
Software decoder uses only ~7% of my cpu. Rest was consumed by x264.
In my example 100 min movie would be encoded in about 400 min.
So with hardware decoding whole process would take about 372 min. So basically you save half an hour. Frankly I don't care because I do my encodings when I don't use my PC. Mostly during the night so I wouldn't see a difference.