After some research on downgrading GT5 from 2.17 to 2.11, I think this should work:
1- Delete your Gran Turismo 5 game data, along with all savegames
2- Start GT5 and let it update to the 24th out of 27 updates (which is 2.11), allow the 24th update to finish installing but CANCEL before the 25th finishes downloading
3- Allow GT5 to install game data normally
Note that you will need to stay logged out of PSN whenever you start GT5, lest you are pressured into installing 2.17 again.