Dear Will:

"Will" wrote in message
...
....
If air flow is
badly impeded by this

It will be.

then I could remove the original filter
material.

Then the "muck" you vaccum up will build up on the fins of the
"fan", changing its dynamic charateristics, and ultimately
causing it to fail. This is a bad thing. Instead, increase the
surface area of the external "bag".

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

Yes.

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.)

Likely the answer to your question is "no". Because you are
trying to push that same volume of gas+ through a smaller total
orifice, so the average speed must go up.

Also, some blower motors are cooled by the exit air stream. So
be *sure* you do not constrict this airflow for long...

David A. Smith