Eighty knots: As I'm sure you know, the S fuselage has a radius on the corners of the box beam structure of the fuselage (as opposed to the right-angled corners on the previous models). This is inherently stiffer, though I can't quantify what the improvement is. This design also has twice as many longerons (eight cf. four) relative to the XL. Again, another inherent improvement. So the only reinforcement provided are a few plates around the tail-wheel segment itself to take up and distribute the concentrated forces here. The mains are also shifted forward and, in the factory version, these points get some reinforcement too.
Petr: There is a kit available, but they are very reluctant to provide this for the XL. You'd be best speaking to the local distributor if you wanted to go down this route.