With the new filter the restriction gauge is reading less than 8. As far as I know it is working fine. Getting to the top of the engine is a trick. I have to clean out the closet and pull the floor. Than I need to worry about the grime gatting in the bedroom. Being fulltiming and DW with me all the time I don't know if it is worth the trouble. I think that trying to fix what ain't broke is not a bright thing to do. The engine seems to be breathing fine.
Is there a filter that passes more cfm than the one you are using? But, as you have stated, if your plumbing is not resized it will make no difference.