Its really dependent upon the engine, the intake, etc. Some dual planes are even cast with a hole in the bottom if the divider.
I prefer blending. I tried to grab a screen shot showing some guys just make a notch, others go the whole way across, etc. Again we arent talking huge numbers. I definatly wouldn't go nuts with this idea with an angle grinder in your garage without any real way to test the before and after. A spacer is reversible...grinding isnt.