The tank screen should be pure white. If it is discolored then replace it.
A new O ring should accompany a new screen. 928 International sells new tank screens for your car.
A discolored screen can fragment and cause small pieces to go directly to the pump and cause the pump to die.
The metal sleeve is glued into the tank. It is also tagged in place with a couple of plastic tabs. Gas should not leak between the tank and the sleeve. If it does the tank is bad. There is a glue that can be used to re-glue the sleeve into the tank - although I do not have the immediate reference of the correct glue. The sleeve can be removed if it is stripped by using a heat gun or hair dryer to expand the plastic around the sleeve.
If the sleeve is in good condition then the new O ring should fix the problem. I use some Teflon paste on the threads to insure a tight fit and to make sure the threads will not get varnished up by old gas.
The plastic tank was part of the 928 being a world class innovation. If you look at the materials used and the science at the time you will understand that Porsche was a pioneer. Today many of the materials Porsche pioneered on the 928 are part of most production cars. A plastic tank will not rust out - a major concern for Porsche with previous cars. A plastic tank will not split open on impact - remember the Pinto. A plastic tank allows the tank to be molded in a very tight configuration so more gas could be carried. Imagine what a metal tank would look like it is was to fit into the same space. The tank also has a molded in reserve tank that acts as a baffle in tight turns. All in all not a bad design.
If you are in there anyway and are finding so much rot and dirt you are probably better off pulling the tank. The external insulation on the gas hose going away from the tank is there to protect the hose from chafing.
The hose can get old and crack and can wear at critical places. A gas hose can be fabricated out of high pressure injection hose. The insulation can be found at the local Home Depot. If the O ring is old and crumbling it is a good bet the other hoses in the system are rotted. There is an equalization hose going from the filler side of the tank to the main part.
It is fiber coated hose and runs through the frame of the car. There are also vapor recovery hoses near the filler.
With the tank removed you can replace the carrier padding. These are probably bad too. The padding prevents the metal straps digging into the tank and causing future leaks.
Dan the Pod Guy