Dahon offers a bunch of different stems. All these stems are interchangeable, i.e., should work on any Dahon.
The Fusion2 now has a V-Clamp from outside. It comes in different length, telescoping and non-telescoping. The ones on the Jetsteams are shorter and non-telescoping. These are the stiffest.
I did not do an extensive comparison, but from some test rides with other folders, I think these Dahon stems are stiffer than all other folding stems, including Birdy.

If you have a telescoping post, I recommend an ahead conversion (chop of the QR and use a reducer), see this pic: