We normally run defoliant with a ground rig.  I use 10 gpa, some around here go 15 even 20 gpa.  I haven't seen that much difference with the higher gpa as I have with upping the pressure.  Currently running 11002 AI tips @ 95 psi.  (I would like to use regular flat fans but I have sensitive areas that I must slow down and let the AI's do their thing to avoid drift, at least on the outside pass or two.)  We always felt we got a better job with less chemical using a ground rig vs a plane.