I also filter stout to remove yeast and break material for a flavour improvement. The stout will be much smoother and cleaner when filtered.
Filtering isnt all about apperance!
A teaspoon of napisan in tap hot water soaked for 15 min, rinse and replace...... soak overnight and the filter comes up as white as the day you bought it.
I use Brewbrite then cold condition and fine as normal. Stout is not generally opaque, when you raise the glass against the light you should see lovely garnet highlights like looking through a Church Stained Glass Window.