The Porsche looks great but I think the scanned background lets it down - I think it might be alright if it was out of focus or moving as part of a video but as it is it looks very much like a scan…
I get what you’re saying with the scans. They are not superb in that regard. Although I’ve used quite shallow DOF focusing on the Porsche which in my oppinion almost makes it work. Even shallower DOF didn’t really make sense in these scene setups.
I will try a motion blur setup with something different later on when I get the time
It almost looks like you’ve composited a picture of a real car into a real-life miniature model of the environment, which is a compliment on how good the porsche looks!