How much time one can gain with custom setup vs. default?

Days

depends on car/track combo.

Custom setup are always better than default...

Let me put it this way - what BL gp / XFG WR would be with default setup?
Give me your guess.

I can't say about XFG, but i couldn't even do 1:14 time in FBM and my pb is 1:11.92

Thanks a lot CC, it gives me an idea how much I have to improve before even trying to tweak the settings.

I would rather try a custom setup than using a default one, because the default setups aren't very good to drive. Try downloading some from setupgrid.net.

or
http://setupfield.teaminferno.hu/