Norwegian are saving 400kg on every flight made by the modified aircraft. No doubt someone with access to tech data will be able to calculate what that means in terms of reduced fuel burn. The only calculation needed is the cost of the refurb (including AOG time) v fuel saving. By adding in cost of capital, you will get a payback period, and management must judge whether that suits their financial objectives.
The weight of passengers is a red herring. Their weight will not be affected by Norwegian's seats!