I guess part of that study referred to frankfurt where there was a distinct capacity difference between VMC and IMC operation back before the new runway opened. Since the runways where too close to each other to allow independent operation they had to separate aircraft approaching alternate runways still by the normal separation, however in VMC they could ask aircraft to maintain own separation, only condition was that they do not overtake each other.
From what i see that is no longer the case since the new northern landing only runway is independent of the other runway used for landing. Haven't been asked to maintain own separation since then at least.