Matt,
From what I know from the documentation, the less you 'SET' the better the performance. I specifically remember this about ORDER. I also suspect that rushmore exclusively recognizes .CDX, and simply disregards .IDX. In fact, if you want to increase performance, and robustness to a certain extent, I'd advise to changed the .IDX into .CDX tags.
HTH,
Marc
If things have the tendency to go your way, do not worry. It won't last. Jules Renard.