Okay, I weighed the factory parts.
Forend is 9.75oz. Accessory kit is 2.3 oz. The kit includes three five groove rails, hardware to install them, and a little torx wrench. So maybe 10.5oz with one rail and a bit less than 12oz with three attached. I don't have a stock forend in front of me so no way to compare them. You can mount your accessories top, bottom, or lower sides. It looks like Benelli took plenty of time making it as light as possible while keeping it strong.