I got my GT5P one week ago, and I'm used to the default bindings, but found the DualShock3 controller to lack in sensitivity (maybe just the one I have) so changed to the bindings with the L2 and R2 as brake and throttle. But that was just too difficult to getting used to for me so my bindings are like this:
This might seem very very strange setup, but it works perfectly for me 👍
R2: Throttle
Square: Brake (this button is sensitive enough to be able to regulate the breaking)
Cross: Shift up
L2: Shift down
Then I have,
L3 stick: Look right and left
D-Pad: Steering (ye, I'm really bad at steering with the sticks, so I'm staying with the D-pad and yes, that's not very optimal

)
EDIT: Can you use the SIXAXIS ability to steer?
I also wish I had money for a wheel and a nice stable setup for it - maybe sometime in the future. I'm saving up for a HDTV at the moment, to replace my 20" 4:3 TV
Would be great to hear some tips from the top racers here at GTP that uses a controller 👍! Can't find a thread about it