No - it's locked to 900, but the adjustment is not needed. GT5's steering is most sensitive in the first 360 degrees of motion - 180deg either side of centre - and beyond that the sensitivity drops quite significantly. What this means is that although you have the full 900deg rotation (450 either way, 1.5 turns) available should you require it, you rarely need more than half a turn of lock (180deg) to make any corner in the game in any car*. For all intents and purposes I guess you can say that the game runs in 360 degree mode 99% of the time and switches to 900 during the 1% in which you dial on more than half a turn of lock.
*Suzuka's hairpin, for example, only needs half a turn at most - even in the X1 - and that's one of the slowest corners in the game.
tl;dr GT5 sets the G27 to 900 but it drives like it's in 360