^putting a tire on a wheel within the approved rim width range is not stretching a tire. Putting it on a the wheel above the approved range is stretching.

Absolute rigidity is not absolutely better for handling. Road compliance is important too. Maximum grip on a skid pad does not necessarily mean better lap times.

And what is "worse" slip angle? What is that supposed to mean?

fwiw, it's optimal for the rim width to be about the same size as the tread width, which is usually the upper side of the approved range. But it's not stretching.