Is it unknowable? Couldn't you take a population of recent studies (because IIRC, the field has never been as homogeneous as it is today) and compare the reproducibility of non-political studies with political studies? Obviously some care needs to be taken with how we code "political" and "non-political", but I would think that could at least give a pretty good indication about the effect.
It would also be interesting to repeat the experiment with studies from different points in time (when there was more ideological diversity) to see how the effect varies with homogeneity. My guess is that if there is an effect, it varies nonlinearly with diversity--in other words, a field with 90% political homogeneity might have an effect that is ten times that of the same field when it was only 70% homogeneous. I suspect that once you hit 90%, dissent is much more effectively suppressed than at 70%. But maybe not?