Do a search on this forum or the cen one for genesis diff oil or something like that - theres quite a few threads out there with different opinions on what weights to run from guys that race, thats what I did to research it before I did it. I'm not racing so I don't care if I lose a bit of performance, everyone has a different opinion on this as everyone has a different driving style, just give it a shot, its not gonna hurt it if you get the weights wrong - you can always change the oil, it doesn't take that long to do the front and rears.
Locking the centre WILL wear the drivetrain quicker, I'd say lock it - you can always put the diff internals back in if its costing you too much.