On my ABS K75S there is very little of the brake lines that could be replaced by SS lines, at least on the front. Coming from the brake lever, only about a foot is flexible hose, then it switches to solid pipes, like on a car, all the way under the tank back to the ABS modulator, and all the way forward again to the radiator, where you get another foot or so of flexible hose to replace.
I've never done a before/after comparison, but my expectation is that with stainless the brakes would get a less mushy feel, since a regular brake line can balloon out a tad under braking pressure. But with the massive distance the oil has to travel on our ABS bikes, we're probably always stuck with some mush.
