Yes a TT is less likely to be similar to a Beetle underneath than say a Golf. Yes I'm sure the spring rates, shock tuning, alignment, tires, styling are different. Using your analogy, either the ballroom dancer is wearing boxing gloves or the boxer is wearing dancing shoes. Given adequate training I'm sure either could make a convincing imitation of the other, but what would you rather start with as a matter of principle?
Lets have a boxer for boxing (big frame, muscular), and a ballroom dancer for dancing (lithe build). There's product differentiation, and then there's whole-new-product. I draw the line at a luxurious drop top having the exact same interior as a hot-hatch coupe.