i have spent a lot of time testing out and researching the gt5 3d feature on my 3dtv panasonic vt20 plasma. and here are my findings:
settings:
3d feature works really well if you get the settings right,
use 3d strength =9
convergence =6
increase contrast and brightness of your tv screen by 50%.
cockpit view:
do not use the cockpit view, although there is no cross talk at all on my pana plasma, using the cockpit view i get headaches quickly, this is due to my eyes having to converge more to see the contrasting depth of the cockpit coming out of the screen and the background going into the distance, having your eyes converge like this for long period of time will give you headaches. unlike 3d movies where the coming out of screen effects only comes once in a while.
3d glasses affecting accuracy of face recognition:
although in arcade mode and practice mode in 3d, you can use the headtracking with the ps3 eye, the accuracy of the tracking is dramatically reduced due to your 3d glasses covering your eyes. the tracking technology used in gt5 is called face recognition, but when your eyes are covered by the black lcd glasses, the ps3 eye struggles to recognise your face. so my recommendation is not to use the ps3eye when using the 3d function and vice versa (note that head tracking only works in cockpit view and as i mentioned this gives you headaches when seen in 3d, so that is another good reason not to use both these features at the same time).
headtracking:
this is especially useful when playing rally racing or drifting tracks where the direction of you head and the front of your car are not always parallel, ie. you want to face the direction you are sliding. headtracking is also useful in racecars where the windscreen view is quite limited due to the livery and rollcage, this is where moving your head around to see through the gaps is useful.
other advantages:
other advantages of headtracking mode over 3d mode is that you can play it at 1080p plus use IFS to increase framerate to give an overall better picture quality than the 720p 3d mode.
general use:
most of gt5 does not allow for the use of the ps3 eye anyway, so in those instances i use the 3d function, but when i play arcade mode i use the ps3 eye headtracking with the cockpit view.
conclusion:
use 3d mode with bumper cam for all gt events and online
use 2d mode with cockpit view with ps3eye headtracking for arcade and practice especially when using race cars, rally races, and drifting.