I have some into possession of a 600 8 sp. rear derailleur (double), and it seems like replacing my RSX 7 spd. RD (triple) would be an upgrade. Two questions:

1. The RSX RD definitely has a longer cage. Are they interchangable? I was under the impression that a rear derailleur was a rear derailleur. Aside from chain length considerations (I might have to knock a couple links out), is this doable?

2. Is this, in fact, an upgrade? From my research, 600 became Ultegra, and RSX was more or less bottom of the barrel. Now, I'm happy with my RSX RD and have had no trouble, but I like to tinker. Worthwhile?

Thanks for the help.