It depends what you do with it. If you go diving or fishing, the stability from the flooded hull is helpful. On the other hand, if you want to tow skiers or just get off to a quick start, then block it off.
Haven't done a comparison on fuel consumption, but common sense suggests that it has to be better with a dry hull. Depends how often you stop and start as it won't make much difference once you're planing.
I've tried mine both ways - just about to block it up again. (Partly because the top of the transom was alarmingly close to the water with the hull flooded) Use a small bung in whatever arrangement you go for because some water will get in there. Mine uses a 4" inspection hatch with a bung added to it and a couple of plastic plugs in the air vents at the bow. Use suitable amounts of Sikaflex to seal.
My cables sit on the deck and I hadn't considered moving them. If you go that way, I'm not sure how you'll route everything. From previous posts, Avon don't recommend cutting holes in the deck but you may be OK if they were only 1-2" diameter. Details of the helpful individuals at Avon I think have been posted before.
There are some posts on trunking and cabling etc if you look around. Main thing is to make sure its sealed where it goes through and the trunking comes high enough above the deck to avoid water washing back down. And don't have any cable joins within the trunking!