Stonebound gives you more bonus the more actual durability was used, not percentage wise. It is not straight relation anymore, but it is still better to have tool with higher durability for it.
To compare ardite VS paper/thaumium for a given rod/binding, one modifier slot gives max of 4.0 speed with full redstone, but stonebound 2 gives the potential for much more if the tool has enough total durability.