I don't consider the shiver an upgrade. Similar quality damping, just inverted.
What year 888. 2004 the R was the top of the line using the best internals. This year there is no RR this year. You have choice between the Rc and RF (god I hate marz this year, I think there is another in there somewhere) You fork has one leg hsvc and the other ssv (or the other crappy damping).
the upgrade would be noticeable but to be honest not fantastic.
Boxxer WC is ok, but is still based on old tech. Depending on who you talk to this is definately not an upgrade. The FOX Dh...you must be rich
Personally the 888R (or rc this year) stand on top for racing with the shiver, Boxxer and fox dh (well has anyone ridden this thing yet?) I don't see very many places for you to upgrade.