My engineering friends (hunters, but not gunsmiths) used to tell me, that the important thing is the size of the wood-to-metal contact area, which in sidelock guns is potentially greater than in boxlocks. So, the actual impact per square inch of wood, considering the way the locks and action come far into the grip, miight aclually be less than that of a boxlock - provided the fit is perfect at every point of the contact area.

Russian Hunting Shotguns