Here are some settings for the G25 I found on the F1 2013 technical assistance forum (Community Codemasters). All credits to "Ahorn" who made and posted them.
Original website (post and pics are in german but easy to identify/understand):
http://community.codemasters.com/t5/F1-2013-Technical-Assistance/Einstellung-für-F1-2013-PC-und-das-Logitech-G25/m-p/365512
Logitech Profiler:
Global Settings
Overall Effects Strength = 100%
Spring Effect Strength = 100%
Damper Effect Strength = 100%
Enable Center Spring in Force Feedback games = Unchecked
Center Spring Strength = 0%
Report Combined Pedals = Checked
Degree of Rotation = 271°
Allow Game to Adjust Settings = Unchecked
Game settings
Use special force feedback device settings = Checked
Overall Effects Strength = 100%
Spring Effect Strength = 100%
Damper Effect Strength = 100%
Enable Center Spring = Unchecked
Center Spring Strength = 0%
Report Combined Pedals = Checked
Degree of Rotation = 264°
Use special game settings= Checked
Allow Game to Adjust Settings = Unchecked
In game (F1 2013)
Advanced Wheel Options:
Steering Deadzone 1.50%
Steering Saturation 8%
Steering Linearity 25%
Throttle Deadzone 10%
Throttle Saturation 3%
Brake Deadzone 10%
Brake Saturation 3%
Also don't forget: Override input device : Steering Wheel
Force Feedback
Environmental Effects 70%
Forcefeedback Strength 80% (60%)
Wheel Weight 70% (50%)
I also use the F112 Real Feel Force Feedback Mod.
These are the best settings I found so far but I think that there is still room for improvement.