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The stock seats sag because GM didn't put anything between the foam and the seat springs. After time and use the metal wire springs cut their way through the foam and the seat feels bad.
The foam itself also breaks down and loses its ability to maintain its shape.
As a result of these things, the seat feels flat and unsupportive.
Give Matt at Vette Essentials a call. He can set you up with a set of new seat foams to go along with a set of new leathers. When installing the new foams protect them by placing some fabric over the metal wire seatsprings so the springs cannot cut themselves into the new foam.
The result will be seats that look and feel brand new, and last much longer than the OEM ones did.