One must be aware that there is a difference between air flow and air pressure, even though their is often a relationship. In the vacuum balance adjustment, we are really looking to balanced air volumes going to each individual cylinder. Given they are all equal in construction, the pressure can be used to infer this balance as when all mechanical factors are equal, so should the pressures. Balance tubes would essentially allow the gauges to lie about actual air volume flow in each carb by contaminating the otherwise sole source pressure readings.
One of the reasons why we can use slide style carbs on the SOHC4 is because the intakes have vacuum pulses, rather than a steady vacuum level.
Steady vacuum places side loads on the slide making them difficult to move up and down and would wear quickly if forced during this condition. This is why most auto carburetors have butterfly style air regulators, rather than slides, working against the common vacuum from 6 or 8 cylinders placing an average vacuum level against the air flow inlet restrictor.
I would think that for the vacuum balancing tubes to be truly effective in averaging pressure, they would need to be rather large tubes to allow air to flow rapidly among ports. But, then rather than laminar flow going into the intake valve port from carb, there would be flow diverted and alternately mixed with turbulent cross flow from other ports. At High RPMs/high intake velocity/volumes, such turbulent flow would wreak havoc on cylinder chamber filling efficiency.
Commonizing the four intake runners would essentially make vacuum balancing totally irrelevant. And, a method for assuring the identical mechanical slide position in each carb a requirement, in order to have all the individual carbs providing equal contributions. I would think that slide wear/movement issues might also surface with such a modification.
