I doubt you would notice any difference in fuel mileage... An alternative to locking out the vacuum advance is using one of the adjustable vacuum advance canister's, by turning the adjustment you change how quick the advance comes in. Sounds like the engine likes the advance, it would be worth a try to lock in the vacuum advance arm again. Lighter springs and the different arms will also bring the mechanical in a bit faster.... I would suggest just changing one side of the advance at a time, messing with both at the same time can lead to a lot of frustration!!!!