David Barnes, no, I did not notice a further improvement. But hanging a closed volume off of the manifold will create an acoustic resonance that may (or not) have an effect. Why take the chance. I also wanted to minimize heat transfer back to the intake with the crossover in place. Finally, I had some vacuum leaks that were difficult to treat because the crossover gaskets are hard to find and needed replacement.
I did not notice much difference between wiring open the butterflies and removing them. But removal leads to a cleaner appearance.
David Potter, the butterflies are controlled only by the position of the throttle linkage. You need to step down hard (over half way) before they begin to open. A second benefit is a marked reduction in the pedal pressure when the extra linkage is removed. This by itself makes driving more fun.
Finally, some input about spark advance would be appreciated. SNG Barrett sells a vacuum advance that will fit on my distributor and perform as in Series 1. But I do not know if that is the right choice with dual Strombergs instead of triple SU's. Seems to me the best answer to all these issues is to retrofit a Series 1 manifold. Anyone got a spare $4K?