I’m running right around 98% singulation and spacing with a stock 9523 White (White meters).  

Some seed does better than others.    Any rounds are the best.  Small flats are the worst.  

Main thing is to watch the skips, you’re better off with some doubles than a whole bunch of skips.  

As for spending $3000 per row to put PP stuff on it to gain 2-3% on spacing and singulation.  Do your own homework on the payback.  Takes a lot of theoretical lost bushels to pay for that stuff.  

If you don’t have them a pair of EZ Air remote controls for your air pressure control valves is a really nice addition to a White planter.  Once in a while used ones show up in the classified or on Big Iron.  I think new ones are still available also.  https://deiteringengineering.com/ez-air.html