I'm wondering about this myself. Part of me wants my garage to open up completely in GT6, and mine has well over 1600 cars! But at the same time, I'm all about starting from scratch, or close to scratch. I did do the money boost in GT4, and I wanted more, because there were some choice cars in the Used Dealership I didn't see again for a good long while.
This is another one of those reveals I anticipate learning about in E3.