Adding mass will change the inertia of the gun, which may affect reliability as the Benelli is an inertia-operated weapon.
However, it seems that most of the reliability problems with heavily accessorized inertia-operated semi-autos involve a combination of a TacStar sidesaddle and a Surefire weaponlight. Both items have considerable mass, especially the sidesaddle when fully loaded. An Aimpoint Comp doesn't have much mass, even with the appropriate mount -- I have a Comp M-XD with the Aimpoint Rail Grabber on my Bushmaster and the entire unit is still very light.
Bottom line, I don't think the Aimpoint will affect the reliability of your gun. If it does, contact Wolff Gunsprings (http://www.gunsprings.com) for a special spring to solve the problem.
Justin