Some modders would tell you they don't want their mod balance changed by anyone and configs are only for end users personal changes, other modders actively balance against each other much to a lot of peoples annoyance (Railcraft, Forestry, IC2, ect.) if they don't use the other mods. I've never heard one say they put the configs in so a modpack maker could just change things to what they thought was fair, as a experienced user of some of the mods I could tell you a lot of them really aren't needed or were flat out bad ideas.Go ask the mod devs. They will tell you default configs are balanced against only vanilla. Thats why they give config options. So that their mods can be adjusted to not invalidate a different mod.
E.G; Look at the Big Reactor's change here, 66% power increase. Why. It was fine as it was and I say that as someone who actively pursues the easiest power sources maintainable. My Big Reactors were producing almost 10000RF before this change, now they got so much power output I don't even know what to do with it all. I could run quarries as far as the eye can see with 16600RF. I could pull up so much fuel with all those quarries my reactors could run until the day the sun burned out.