I have just been and tested, f430 scuderia / sport hard tyres on the Nordschliefe, as I have not upgraded my T500 firmware yet and wont until I find some good reason to - so far it seems to work perfectly...
first I ran a lap with my normal settings (simulation : power steering on : force strength 4)
next I changed force strength up to 10 and ran another lap, the steering was a bit heavier and forces when riding kerbs or oversteering a bit stronger, but not massively so, maybe 1.5 times as strong?
then I changed force strength down to 1 and started another lap, the steering was much lighter and lacking in feel , even compared to strength at 4
Because I think only premium cars have the FFB working correctly. The other ones don't have that support.
from that test I would say the 1-10 scale is not linear, 1-5 is much more of a difference than 5-10 IMO
I hope this helps you in some way