While I don't think Marx's critique of empiricism sits well with taking (as does Sraffa) physical quantities of things as primitives--indeed not taking things but rather the (labor) processes of which things are temporary results as primitives is what Marx commits himself to his critique of empiricism or thingification or reification (see my previous quotes from Theories of Surplus Value); his 'metaphysics' is more process than substance based, though of course the differences between process metaphysics (Rescher) and materialist dialectics (Levins and Lewontin) have to be explored--I think the TSS school has made the opposite mistake in ignoring the importance of physical things, the importance of physical quantities on the accumulation process.
The surplus is indeed at any point a collection of physical things, and Sraffa's theory is much better at capturing that than Marx's value focused transformation tables.
Capital is process and things. Duality.
RB