If the only standard for a diesel is 20% opacity then you have nothing to worry about. A direct injection engine, like a TDI, couldn't possibly fail that test and still run well enough to move the car around. Some counties in Utah have the same test. I think it's done just to make diesel owners jump through some hoops. The test itself is completely meaningless to a TDI. The standard is set at 20% in order to allow old-fashioned IDI diesels a reasonable chance to pass the test.
-mickey