Why should it improve something that isn't a problem ?

The fork is stiff enough by itself, no lateral stability problem.

What seems sure is that it will increase the weight.

As for the Brompton, there are many useless, if not not working well third party custom components for the Birdy sold at an outrageously high price.

The basic concept of this front suspension is questionable: if the two springs do not have exactly the same stiffness, they will tend to create a distortion of this suspension.