Marx did not understand capital to be productive in the way Joan Robinson or Herbert Marcuse did. See discussion in Geoff Pilling's The Crisis of Keynesian Economics.
What Marx meant by capital fetishism is key. For example, if a Christian says God is a father, a son and a Holy Spirit, he does not mean this in a metaphoric way. He means them to be true and simultaneously true! They are treated not as metaphors but brought into metanymic relationship as mutually interdepedent signs. Such mytho-logical statements of course conflict with logical rules of ordinary experience but they can make sense in the mind through religious experience.
Now I think what Marx is trying to suggest is that the metaphors by which we understand capital (like land with a scarcity value, like gold that is intrinsically valuable, as a machine) are all treated as metonymic signs that are true and simultaneously true. Capital *is* all these things, and these things simultaneously. So our understanding of what capital is non-sense, though in everday bourgeois affaris--the religion of everyday life as he called it--such nonsense can make sense 'in the mind.'
Oh well, this would have to be REALLY developed.
ANother note: no one has ever written greater paeans for the productive powers unleashed by the capitalist system. This left Schumpeter in awe in his essay on the Communist Manifesto, and Schumpeter did not even have access to the tantalizing passages in the the Grundrisse on the effect of relative surplus value on the qualitative diversity of use values produced, the stimulus provided to natural science, the challenge to superstition and tradition. Suffice to say, the marginalist's apologia for the capitalist system pales in comparison to how Marx could talk it up. Schumpeter had the integrity and learning to realise that he had to take up both Marx's praise and critique of the system. So he would only play around with static marginalist principles of substitution for which he really tried to substitute his idea of new combinations, new powers, new goods as the foundation of his dynamic economic system. But this idea was already developed in Marx.
Yours, Rakesh