Im not sure(I think you typed 25 instead of 27?) about that because it is most useful for me to just map the RA function to the controller, this is for both the G25 and G27. But as for the jump from GT5p to GT5 concerning the G27- this is the ludicrous statement Dev made- I think more buttons work on the shifter, at least without trying to figure out which buttons on DFGT screen match up to the G27's actual buttons. If that isn't correct, my bad, but from when I first hooked up the G27 to the 2 games, again, this is if my memory serves me correctly, more things work in GT5 rather than GT5p.
Besides the buttons, sequential shifting can happen in GT5 and that wasn't possible in Prologue. This rendered the F2007 about useless and made many other cars very unrealistic to drive- because they have flappy paddles in real life. Nonetheless, the G27 is much more useful in GT5 than in GT5p.
For clarification, I enjoy driving, not button pushing. This means that if a wheel can perform better when it comes to actual driving then it is more useful. Also, the RA function is good for two things: brake balance, which only needs to be adjusted during a race under circumstances of extreme tire wear difference between the front and rear, and using TC as a launch control, also only needed in rare circumstances.