Oh, the setup you mention is more than possible. It's used on a daily basis in both the wind turbine industry, in commercial car development and very much so in the racing car industry. And many more.
Point is, these companies probably have substantially faster computer hardware than you have and they do this all the time. This makes them able to use fine, optimized, meshes and also to be aware (through experience on each particular setup) of the impact of simplifications applied to the CFD-models.

CFD is playing a key role in a broad range of industries, only mentioning a few of them above.

So the answer to your question "is CFD useful for testing cars or planes?" is a loud YES, and it has been for quite some years now.

You have a point, though, that CFD is often used as a qualitative tool more than a quantitative one. It's easier to get contour plots and streamlines from CFD than correctly predicting lift and drag, within 1% of experimental results, on a stalled airfoil.

BR Mads