Yes, a side plated gun does robably have an advantage over a plain, standard boxlock, IMO. Boxlocks, due to their design have very little wood contacting the action and are prone over time to develop cracks and split open at the head of the stock. A sideplated gun, with a screw running through the weakest part of the stock, clamps all this wood together and strengthens it. Same principle as a C clamp. There is a trade off to accomplish this though and it may cancel any advantage: Wood has to be removed to inlet the side plates and that does thin wood in a critical area.
