You can have both.
Series mounted motors give you lift, ie suction.
Parallel mounted give you less suction but more
air moved.
In theory you could have 10000 CFMs, but without
sufficient lift to suck the water into the wand, it would
be useless.
I can't remember where i saw it, but some time ago i
saw a table which recorded the CFMs drawn through
a wand at the carpet interface, surprisingly enough
there was virtually no difference between AO ans VO
machines.
AO had less suction to draw the air through the carpet fibres,
VO had more suction so could draw more air in.
Which just proves the point, they all do the same job.