There will be no binding action w/ stiff bushings. In the event of body roll or hitting a bump w/ one wheel etc., leaf springs simply twist. It's part of what they are made to do, and the reason why a leaf-spring car doesn't need much rear sway bar...the leafs are already (by design) providing roll resistance by twisting. (In fact you might save the weight/cost of having to use a rr bar by using stiffer bushings, depending on your setup.)
Binding is what happens when you have two conflicting types of suspension links working against each other...which you may or may-not end up with here, depending on who's advice you are going with...
The downside of stiff bushings is increased transfer of vibration and spike loads into the mounts which may become more prone to cracking, and of-course noise...for that reason I would hold the line at plastic and not go any stiffer.