In my (humble) opinion, the amount of difference you notice will depend upon what was on there before. If you had an enclosed unit with the snorkle on it, simply turning the lid upside down will make a difference, Going from that to an open filament will be a big improvement. From there to a K&N - probably better, but the big advantage is that you are through buying air filters.
Notice the lid on junkyardjohn's. It also has the K&N lid. Bet that helps a lot.
The engine is an air pump. Air in - Air out. The more the merrier.
over-simplification - but it sounds good)
I run a K&N on my Grand National. Big difference over stock. Saving my allowance to do the same thing on the truck.