If You are running LO on vista and you've installed it on C: drive in prgrams/files, then OS is preventing any manipulation with files in there.
You gotta make copy on other drive and than you'll be able to apply mods.
As to fuel usage, it ruins any of my missions when using moded plane.
For example, if i use mod of f-16 on mig-29g base and i fill my and wingmans plane with 9 tons of fuel, he still calls bingo fuel after just few moments.
Well, thats mods for you i suppose :)