I wonder why these pumps come without foam covers in the first place...
Maybe everyone here should complain at the manufacturers (or distributors and stores) so they start putting a foam cover into the box. Can't be that hard or costly.
At least they should offer it as accessory.
The power reduction shouldn't be a problem as you should clean the covers often.
Personally I am a gambler. As ca1ore mentioned, foam covers gunk up so quickly that you're really looking at 35% normal flow after a day. It would drive me crazy to have to clean off the covers so frequently.
To date I have escaped disaster.