simulation, power steering on and strength 6 for my DFGT.
simulation, power steering off and strength 8 for my G25. (although I haven't run the G25 with prologue since spec II)
Power steering is something a bit odd in prologue, at least on the DFGT. If you turn the wheel at a fast enough speed you suddenly feel less resistance to your movements so you can spin the wheel faster. Otherwise it doesn't seem to have any effects. It's handy for turning into sharp hairpins.
I don't think it's needed on the G25 because that has a little bit less resistance than the DFGT and doesn't really get in the way ever.