Interesting about the shot used on top of the insert, I haven't tried that.  As well as the insert modification.  What type of modification?
Regards
Ken 
Ken, Jay Menefee down at Polywad shows photos of the spreader discs with holes punched in them.  I experimented with a one-hole paper punch, cutting out 3 little semicircles around the periphery of the disc.  Tried that vs unaltered discs vs discs one size smaller (20ga in a 16ga reload, for example).  Found that my modified discs gave me the best spread while eliminating the weak center.  Mike Campbell is one who's used the layer of shot on top of the disc and can tell you more about that.