I honestly don't know exactly what valving is in my coilovers. I do have a good idea what valving is in my shocks. I even helped valve both setups...LOL
Spring rates are important but not near as important as valving. I doubt anyone who has a OUO kit even knows what kind of valving is in their coilovers. But what I can tell you is it will either make your truck drive like complete garbage or real nice.
I do know that the valving you get from calling up a shock manufacturer and expecting them to get it right most likely won't happen. It probably won't be bad, but I doubt it will be great either.
One thing you could do if you are fabricating a kit for yourself is run the coilovers with very little valving and then pick up a set of bypass shocks....then tune it yourself. Otherwise, it isn't a whole lot of fun pulling a set of coilovers apart over and over to get the valving correct. Then remember that the majority of your ride is controlled by the front suspension, even if I tighten up my bypasses and make it ride stiff in the rear, it still rides better than a truck with a set of fixed length traction bars on the back...LOL So really, it may not make much of a difference overall....