Fuel Filter Sizing

Perhaps my comment was too brief for clarity. If so, here is probably more than you ever wanted to know about fuel filters. Think of the fuel delivery system as a graduated set of filters beginning at the fuel dock (we hope) and ending with the engine filter. The coarsest at the dock may be a 30 to 50 micron followed by a strainer on the tank filler (not sure if this is on the 320) then a very fine strainer (usually 30 miron) on the fuel pickup in the tank. Then comes the "primary" (30 to 10 microns) and finally the engine filter almost always a 2 micron on diesels. Each filter catches the contaminants in it's size range. The purpose of the sytem design is to prevent engine damage, nothing more or less.
 With the graduations in micron size, filter life is extended, no filter is overloaded in normal operation and fuel flow is maintained even with significant levels of crud because each filter does only part of the toal work load in the range it was designed to work in. It should be obvious that if a small micron filter, such as a 2 micron, is on the front end of the system, it does all the work in catching all the contamination and in even a minor fuel contamination event would quickly become clogged. With a 2 micron filter as a primary, the secondary engine filter becomes superfluous because no 2 micron particles ever reach it. This is because unlike oil filters, fuel filters have no bypass. When they clog, they shut down to save the engine.
Looking at it another way, if you have a 2 micron primary, it has to catch anything from say 2 to 50 microns. The 2 micron secondary does no work. If you have a 30 micron primary, it gets the 30 to 50 micron stuff and the secondary gets the 2 to 29 micron particles.The work is shared and both filters perform more efficiently. As a side note, filters become more effective with time because they are designed to use the trapped material as additional filter media, so don't change them too often.
The main purpose of the Racor is as a water separator, which can really wreck a diesel. Filtration is a secondary benefit.
The idea of twin primaries with a bypass valve works, but if you really want to go to the max, set up two primaries in tandem with the first a 30 micron Racor followed by a 15 or 10 micron. This set up is used on fire trucks and other emergency diesels, but seems a little overkill ( after all we do have sails).
In my real life job I've been maintaining diesel municicpal fleet vehicles for 25+ years, so I modestly claim some expertise in this field. I hope this helps a bit.