Think you will find about 1/3rd fuel saving regardless of the extra weight. Carb weights 139kg Opti is 163kg. So its the same as putting a 25 ltr fuel can at the back of the boat.
The Carb is a 1.3ltr but the older "clam shell" models were 1.1ltr blocks.
The weight probably comes from the bigger block, alternator, comprssor, etc etc. Its worth it for the fuel saving, but you will probably find you might loose some grunt as Carb engines are usually rated at above there hp rating Merc 90's usually run at about 94hp, were as the Opti will be bang on 90hp.