GT5 is a 3.50fw game so with 3.55CFW there is no need to modify the eboot. That said, I know the frustration you are feeling so I can't blame you for having a go.
The game does work with CFW3.55 BUT under the following criteria:
1. Properly installed CFW.
2.Properly functioning Console (especially the laser).
3.Properly functioning HDD
4.Authentic and properly installed game.
If you're not 100% confident with any of the above points, that is more than likely where you problem resides.
If you're confident you can trust where you got the game from (original scratch free disc is best), try this: Delete the Game Data for GT5 from the XMB and then switch off your console and unplug it from the wall. Leave for a couple of hours (to drain the RAM).
Restart. Game data should re-install and game should run. If the problem persists. You'll have to delete the backup and re-rip.
Before you do, can you play the game straight from the disc? (Don't even think about a downloaded backup if you're having problems - you've got no control of what you're installing.)
How healthy is your laser? It might not be ripping the game correctly.
Hope this helps.
"We all agree your theory is crazy, but is it crazy enough?" - Neils Bohr