Ungodly lightweight sounds good to me. My Salomon SP3 bindings are ungodly heavy. I highly doubt bindings would contribute to knee pain. It's probably all the abuse you are laying into your legs. If you beat up on your leg muscles too much then the impact will start moving into your joints as the muscles aren't strong enough to absorb it. Hit up some Ibuprofen before and after you go, then ice afterwards. Also stretch before and after your go as well. Do all that and I bet your knee pain will go away. You're lucky you still have snow to run on, it's all gone here.
Any ways, what's the difference between the Relay Pro and the regular Relay bindings? Is it just materials and how customizable they are?