Just how good are the setups found on setup field? Are they perfect, or do they often require tweaking?

Also: Is it true that all the really good racers use almost the same setup? Thanks!

Usually they are the WR sets, however that means they are for one lap (usually), unstable and hard to drive with.

I always use them, but i sometimes tweak them, because tbh EVERYONE wants something a little different from their set, it's up to you if you want to tweak the set.

Most racers use around the same set, but with little tweaks, usually the set is the optimum for that tracks corners and the car, but their are little tweaks that each driver wants, adding a few little traits they want to the car

They're often very good, or even world record setups. However, that doesn't necessarily mean they will perform good in a race, as some of them need a very special driving technique to make them fast.

Completely self developed sets, or WR sets adopted and tweaked to suit your style, will always be the fastest ones for you, and also the ones you will be most consistent in.

Most setups there are 60% suited to the track and 40% suited to the driver who made them.
My advice is use them as a start point and change how you see fit.

Although most sets found there might be a bit extreme in the way they handle you might find you will need to add downforce on the bigger cars to get the hang of the way they handle.

At least XRG setup for BL1 is really bad, there is no dampers installed, car bottoms out very easily, oversteers so much that one really can't push it to corner, can't really say anything good about brakes either, I have no idea what kind of controller setup there have to be to be able to use that set for even half decent times.

the setups usually are the best for these combos. you just need to be able to use them properly, otherwise you´ll need to learn how to modify a setup.

Just beware the setups posted by, shall we say, "less experienced" drivers and setup makers. I've seen at least 2 or 3 on there made by such people who were nowhere near WR pace.