I uninstalled GT6 because I did not have enough hard disk space for the downloadable version. Although, even if I had enough space, I'm not sure if the PS3 will allow another version of GT6 to be installed while another already exists.
My main reason for the change to the downloadable version is that my PS3 started having major problems reading dual layer disks, which GT6 is. Rather than change the laser, I decided to buy the downloadable version with a gift card I received. I still have trouble with it reading other discs like Rock Band 3, but eventually, it does read it and boot RB3.
GT6 got to the point where I couldn't get my PS3 to read it no matter which of the online tricks I tried (holding console at an angle, re-inserting disc multiple times, etc.).