It may be applicable to all years, but is it worth it for all years? I don't think it's that clear.
What percentage of 1998-2000 Boxsters experience IMS failure, and what percentage of 2001-2005 Boxsters experience IMS failure?
No one has the these statistics and without them it is impossible to say if your IMS retrofit is a good investment for all model years. But like the OP said the original IMS was fairly reliable and more so than later IMS bearings, where you retrofit is probably not a bad idea.
