I think we would all agree that a Miata is a "sporty econobox" while an Elise, NSX, and S2000 are more pure 'sports car'...

But where do you draw the line? And does that apply for STU and STO/GT2 as well?

IMO, the Miata belongs in STL no matter what label you try to put on it. It's a Japanese econobox 4-banger that just happens to have a small body and 2 seats.. and a damn good suspension.