Howdy, I'll offer up my opinion as some who's built a few on different boats then fished outta them. Lol in terms of weight, you already have the wood for the floor and bench seat in there, all you'd really be doing is taking those 2 pieces and making 1 big solid piece that's only a little bigger (since the boats footprints bigger at seat height). The main added weight is the support underneath but even with wood, it isn't that much if you don't go overboard.
Now here's how I've made my decks. I'll take off the front bench seat wood, then I'll get a few big pieces of cardboard, (2) 2x3 or any stiff plank of wood longer than the length your deck will be , and a role of duct tap. Make sure the cardboard you have is long enough to go from the stern side of the seat to however far in the bow you want your deck to go. Now the nitty gritty. Take a piece of cardboard, and eyeball your initial cut for either the port or starboard side (your choice). Get your wood, set it across the seat supports. This is so that your deck will be as level as the seat is. Set your piece of cardboard that you eyeball cut on top of the wood and trim till you're happy with that side. Set that piece of cardboard aside and do the same with the other side of the boat. Now that you have your 2 sides, set your 2 wood planks on top the seat frame and lay your cardboard pieces on top so that they're up against the walls of the boat. Doesn't matter if they overlap or if there's a gap in between but overlapping is preferred. Now just grab your duct tape and tape the 2 cut cardboard pieces together and boom you have your first draft template. Lol now take that template and set it directly on the seat frame and do your final trimmings. After you're happy with it, just lay it on your plywood or whatever your deck material is and trace the outline. That's your cut line. Cut on the inside of the line if you plan on adding carpet. I learned that after my deck fit perfectly without the carpet but not with. Hahaha that's my way of doing it. Tried my best with words alone. But other people have different ways
10/10 worth it though in my opinion.
Positives:
Sitting higher on water so better view into the water (cuts down on glare)
Makes it nicer to stand
Looks cool (imo)
Adding storage under the deck. I usually would put my battery up there under the deck.
Negatives:
Added weight
Work
Finding time to do it.
Better news is you usually only have to do it once if you deal it right! Even if you don't, then atleast your templates already done