I think we need to hold our horses with GT5. Even Need for speed: Hot pursuit is unplayable at this moment, so i think that we jailbreakers are not able to play GT5 for some time. I bet that the protection of GT5 got changed the last time there was a delay for the game. Makes me want to buy a second ps3 for GT5 alone.
Only installation may take longer time (but i don't think that HDD is slower than BD, even HDD to HDD copying). Launching and playing won't be slower. It will be even faster because HDD seeking is faster than BD.
The only disadvantage is wasted HDD space. But if somebody want to reduce wasted space, then it's possible to delete original files from BD rip after installation. I've did it some time ago, and most games were working after this cleanup
What I was trying to say is HDD Seek + BD Seek is ideal, using all of its available bandwidth. If you have it on a backup manager, and you install it to the HDD from in game, it will be loading the backup manager HDD data as well as the in-game install data, slowing things down significantly.
HDD Seek + HDD Seek is slower, and more strenuous than HDD Seek alone.