Replying to myself ...

I asked here about HEPA and from the replies it seems that maybe I
could get some HEPA standard filter material and cut it to shape to
fit the final exhaust grille on my existing hoover. If air flow is
badly impeded by this then I could remove the original filter
material.

Q1) Assuming here are no other significant outlet vents on the hoover
then would this work in reducing airborne dust?

Q2) Does air passing through a HEPA filter get slowed down more than
it would when passing through a conventional filter? (Assuming all
other things like area covered is the same.)