The problem was not so much the rear derailleur but Prestige system as a whole. The shifters and front derailleurs were far weaker links than the rear derailleur. The rear derailleur itself was very good, out-of-the-box, and the issues are primarily age related, typically premature wear of the pivots and breakage of the pulleys and sometimes the body itself, though the latter is far more common with the front derailleur. By employing a parallelogram front derailleur and metal bodied shift levers, you've eliminated the weak links that contribute to the perrformance issues. The one thing that will further improve shifting performance is the use of a HyperGlide compatible freewheel and chain. Also, teflon lined, compressionless cable housing wil help, if you're not already using it.