I use a   5400  CIH  drill on 7.5"  for years on  beans.    I shoot for  just under  1 unit/acre-  around  130-135k/acre.
Honestly,   when  1 row  is  a little  goofy on spacing,   the next row (only 7.5 inches away) tends to  make up for it.    I feel   spacing is  not just between  plants here,  but  more on a  square foot/ square yard  basis.      

If you were to single out and count  several  square-yard  test patches,    I'd bet  the numbers of seed per  square is going to be pretty close across the field.    


as far as your  question about  wide opening/slow speed  vs.    narrow/fast speed...   I've gone to  wide-n-slow   more on just about everything that's drilled here.  
Seems like when the 'gate' is narrow,  so that  1.5  seeds  fit  past the flute,    sometimes  1 seed will  sneak past,  sometimes 2.   That's huge..like 100%  swing in population per flute gap.     

But when the gate is open  to  say,  3 or 4 seeds per flute, turning    2x's slower,     then  being 'off' by 1 seed per flute is a lot less  dramatic..       
how the seed  bounces down the drop tube  ultimately   spaces it in the furrow'.     My drill has tapered gates, I suspect most drills are similar...  so  not all  3 or 4 seeds  drop at the same time.. they kinda  peel out of the cup one at a time as the flute lifts them across the slanted gate.