I'm pretty sure the DLC is linked to your PS3, not directly to your PSN account. I created a couple of new PSN accounts on my PS3 and when I started GT5 for the first time on each new account, all of my Chromelines and Edge Camaro were already in my Garage.
I'm not sure if the DLC is saved in the GT5 Data folder or somewhere else. However, I'm pretty sure that in the "Permanent Borrow Glitch" thread, the OP got rid of his Edge Camaro somehow.
EDIT:
Here is the thread. Read the first post. It seems the DLC is saved in the GT5 data folder. However, deleting the GT5 Data folder deletes other things, too. Please read the thread before deleting anything and always backup your data first. 👍
PS - If you do delete the DLC car, you can get it back by following the directions in the FAQ section of that post as well.