I noticed a slight improvement when installing to the HDD compared to playing off the disc after the 1.3 patch was released. I got a Patriot XT USB stick for Christmas and tried re-installing to it. I really can't see any improvement in the load times with it which is a bit disappointing. My save-games are still on the HDD so I don't know if that makes a difference.
I guess the game just isn't optimized for loading off a faster drive.