Just thought I'd kick in some experience on these style filters...
I use the same manufacturer filter on a twin cam Cosworth engine... Just 2 more velocity stacks.. :)
None of the pictures of the filters I've seen on this thread are anywhere near as dirty as we get them on the track.
They are really tolerant of dirt..
Always use grease on the sealing surface, nasty thick grease. I use red-line wheel bearing grease.
We found on the midget would have lean out problems and the fix was to put a aluminum dam around the front of the filter. About 1" away.
If you dyno a engine with velocity stacks you'll see an interesting effect, its called a fuel fog column, it will extend 1 to 2 inches above the velocity stack. (The dyno operator is world class and had lots of experience with formula engines... ) With the engine under power if air blows across the top of the stacks the mixture goes crazy. He demonstrated with the hose from a shop vacume. I suspect if you put a air dam in front of the filter so the air coming through the body work enters the filter form the sides but cant come straight in it will help keep the buffeting down..
Short stacks for RPM & HP long stacks for low end grunt and torque.