Here is my current setup for racing ONLINE, focused on Nurburgring, I have excluded NA upgrades so that I can race in <650HP races (if you fully upgrade you might not have anywhere to race online, non-NA upgrades can be disabled to drop HP).
I use the DFP Wheel.
All Assists are OFF, except ABS = 1 (without it your toast)
ONLINE Tune (458 Italia):
676HP Max (all upgrades except no NA)
1242kg (Stage 3, no glass upgrade)
Carbon Hood
Aero Kit (Wing, Nose, Nose-fin-thingie)
Chassis Reinforcement
Racing Soft
Downforce: 10/38
LSD:
Initial: 8
Accel: 55
Braking: 10 (looking to try ~5)
Suspension:
Height: -10/-10
Springs: 12.9/14.3
Extension: 7/7
Compression: 6/6
Sway: 5/5
Alignment:
Camber: 6.5/5.0 *key change IMHO
Toe: +0.05/+0.20 (looking to tweak this next) EDIT: Front Toe needs to be Negative, misread the description...
Brake balance: 8/8 (higher seems better, 9/9 resulted in problems though)
In general the car feels a little floaty, compared to say, a touring car where you feel glued all the time. I'm hoping to tweak this some more in the next day or so.
Current issues are: stepping off the track slightly, at high speed, is very delicate to recover; occasionally I spin-out in 2nd gear because I don't know the track well enough
I have not tried this in A/B-Spec... I would expect it to do something weird given the difference with online racing...