Don't know about row crops but I was doing broadacre wheat last year @ 16 MPH. This was with a EZ-Guide+ lightbar into a T2 EZ-Steer. I really couldn't see a bit of difference from my normal speed of 12 MPH.

Sprayer is a 218 tricycle Coupe (AKA: North Dakota Cessna) with NO power steering. Had to add an aftermarket leather steering wheel wrap for the foam wheel to have enough traction on the slippery wheel, but then it worked fine.