Just my thoughts and I have been an NA/displacement guy my whole life until now and I could very well be wrong, which is often the case. But vacuum is vacuum. Pressure difference isn't multiplied by multiple outlets coming from the same source. If you have 1 or 4 throttle bodies pulling the same vacuum from the source, it doesn't increase the vacuum with more lines. 4 lines of vacuum at the same level tied into one line should give you the same vacuum as one single line. You are necking it down into a single line anyway to get into the BOV. If it were a volume issue, the bottleneck would be the entrypoint into the BOV. I only have put this into practice once on a car that had such a big cam that the vacuum wasn't sufficient to run the power brakes at idle. So I tried running vacuum from instead of just manifold to manifold plus carb and anywhere else I could find it. The vacuum ended up being the same and equally inefficient at running the brakes, in fact I think it may have gotten worse although I cant explain that.