I personally think it would be great if the refractory sawmill had a third output slot, for materials such as bark added from other mods. I believe the feature would mainly affect modpacks and be very useful in modpacks so there could be custom recipes, such as putting logs into a sawmill to get debarked logs, but if you use the refractory sawmill then you could get bark or any other custom recipe output you'd like. This would give an additional incentive to upgrade to the refractory sawmill, as well as provide accuracy/consistency in the concept of upgrading, with the concept of having a refractory sawmill cut more precisely than a stone sawmill.