I am also not keen on cutting down dividers, they are at a certain height from the plenum base to the carb flange and who knows what strange internal turbulence could be created between upper and lower plenum surfaces with a reduction in the divider height. You'd need a dyno, lots of manifolds and lots of money.
Something i have done that seemed to work quite well at increasing a dual plane manifold's upper rev range without effecting the manifolds performance otherwise was adding a 2" open spacer below the carb.
With this set up you leave the manifold's divider as is (maybe just round off the sharp edges) and each plane of the manifold can draw air from both sides of the carb.
The best thing is if you don't like it you can just remove it and you are back to as it was.
I did this with a stock 2v cast iron manifold that had a 4barrel carb flange (these strange things exist in Aus) and the result was very noticeable and i left the spacer in place.