No one said the shoe had to start out as new.
They could just have better LOD management/creation. The replays are a moot point to some extent, being limited to 30fps and all, but the worst case scenarios are the start of the race and far-angle camera view in-game. Even then, fitting 16 vehicles such that each take up significant amounts of screen is a rather difficult thing to do. So it'd have to come down to smarter use and displaying of polys rather than necessarily pushing significantly more. I mean, just to take an extreme example, if you're in last place and the 15 cars are lined up down the road, the car in first place doesn't really need a whole lot of polys. And who knows, maybe they have a better occlusion culling scheme for the cars themselves (cars blocking cars).
The bigger concern I would think would be RAM.