General TABGears: Playing around with your gear ratios can save you some time. Generally you want to max 6th gear at the end of a long straight, but also balance the gears to minimize gear changes through some section of track can also save a lot of time. Also, if you have lots of lower speed sections you really want good lower gear ratios.
Steering: Lock range is personal preference. I generally have smooth measured steering inputs, so I lower the lock ratio.
Power Diff: Helps mainly corner exit and mid corner on power turn, so play with this to give less wheel spin and max acceleration through and out of corners. Lower = more oversteer on power
Coast Diff: This can help a lot with off power understeer, so reducing this will reduce mainly mid corner off power understeer. However, this can also affect braking and corner stability.
Pre-load: This one has me a little confused, but generally my car is more stable with this on a higher setting
Aero: These cars are very sensitive to aero, so the lower the better. Rear aero can help a lot with braking stability, and obviously high speed corners.
Engine: I always seem to run max revs, lowest possible radiator to avoid overheating and fuel mix at max. I have also started to run the lowest brake mapping, but I am not sure how much this really affects things.