The read/write heads actually write and read
the data to and from the hard disk, so you'd think they'd have at least some
impact on the drive's overall performance! And in fact this is true: they do. Improvements
to read/write head technologies, and related
components such as the head sliders, are key to
allowing increases in linear density, which in
turn affects areal density and hence both
positioning and transfer performance. They also allow further miniaturization of the head
sliders and related components, which indirectly allows faster and more accurate
positioning performance.
However, you rarely hear read/write heads mentioned in discussions of hard disk
performance. The reason it isn't often discussed is not because it isn't important, but
rather because it doesn't change very frequently. There have been only five general
read/write head technologies used in the last 30 years! So in some ways, nobody talks
about the heads because there isn't a lot to talk about. 
Most manufacturers make the
change to new head technologies at roughly the same time. Further, many of the
manufacturers license head technology from each manufacturers. Today virtually every disk
drive sold uses GMR heads; therefore, this is basically assumed and not really a topic for
discussion. This equality in basic design generally leaves only minor tweaks to
differentiate head designs between models.
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