I think it's nice that the suggestion was made with the phase-only coupler. You can certainly save money there. However, I would like to note that this does not answer the actual question. I have the same setup and only want to improve the coupling (and thus the data throughput) between the floors. Maybe later I will feed the signal directly to the DINRail, but not anytime soon. So this question is still open: Is it possible to use the Devolo DINrail only as a coupler WITHOUT feeding the signal? And does this setup bring an improvement in throughput?

Why not a pure phase coupler? There is a 1/4 for the price.