If you use (or can find one) a driving force pro you can lock it to 270 mode pressing 2 buttons and the right shift button on back of the wheel (look online for the exact button combo). I used to use it for certain cars in GT5and it seemed to work great. I have a DFGT now and I don't think there's a secret locking mode but I haven't really looked. Not sure if one exists for the g25/27.
In Gt5, I often feel as if my G25 is operating in 270 mode already. I don't think I ever turn more than 90-135 degrees either way, with any track / car. This is also due to fact that better driving is done with better approaches, needing less steering angles. Usually I'd rather slam the gas to correct a line