Pretty broad questions... depends on many factors. For instance, the stock B12 shock is pretty much crap out of the box unless you weigh under 150 lbs. And by 20K it's probably spent. The same shock tossed onto a B4 will work well, mostly because it's dealing with a lighter weight package.
Better sportbike shocks are rebuildable, so when they start to show signs of performance loss, they can be restored.