I can't comment on the firmware since I am using 721 ATM, but here are CSRE settings that I successfully use on all PS3 games - especially on GT5 - and I will also explain some details for each one:
SEN OFF | FF 80 | SHO 70 | DRI 001 | ABS 50 | LIN 0 | DEA 0 | SPR 0 | DPR 0 |
Sensitivity setting for all PS3 games should be always set to OFF. With "OFF" setting, wheel takes the original "SEN" setting from the game (in the case of GT it is 900 degrees) and does not force artificial steering degree. Result is 1:1 linearity ratio. There *could* be a slight offset of the actual wheel once you set the Sensitivity to OFF (maybe 0,5 degree) but that offset is irrelevant and can also by corrected with *centering* feature available in "---" mode. SEN OFF is the only way to go.
FF80 is more than enough in order to get all the feel you need from your car. Going to 100 as you do will not do any good. You will get too "saturated" FFB effect and loose all details and nuances in both wheel and in-game feel of the physics. I use FFB in-game setting of 8 (Simulation mode, Power Steering Off in in-game options, G25 profiler) and 80% provides me with the best compromise of strength and feel of the tyres.
I run SHOCK on 70 because higher settings tends to shake the wheel too much over the kerbs for my feel and I do not like when I can't *feel* the direction of the kerbs.
DRIFT setting is important one for PS3 IMO because it actually removes a bit of motor-dampening and makes wheel faster at countersteering through hands (when wheel *corrects* itself with added acceleration, as in real life). I use only 1 (2 and 3 have tendency to lighten FFB effect too much) and DRI1 setting gives me best feel.