No need for much tweaking here, Live for Speed is probably the PC sim with most advanced support for input devices. The only thing you might want to do, for more realism, is to set the shifter controls to match the real life car, e.g. use the sequential shifter in Formula BMW, manual H-pattern for XF GTI, paddle shifter for Formula 1, etc.
However, if you want the G25 to force the lock at the end of the in-game steering wheel's travel, you should, like in all other sims, match the lock-to-lock in Logitech Profiler with the in-sim car's lock-to-lock angle. LFS lets you check these values right from the car setup menu, under "Steering".