What is the difference in shifting performance between Campy rear derailleurs? During the past few weeks, I've encountered a couple of times the statement that there is no difference in the shifting performance across different tiers of Campagnolo rear derailleurs. I.e., one gets top-notch performance even from their cheapest offering. Supposedly, higher-end derailleurs are more expensive due to the lower weight and materials used.

Is there any truth to this? Is there any other difference between a, say, Veloce and Super Record rear derailleur other than price and weight?
Performance wise the largest difference is that mid- and higher level derailleurs have ball bearings while low end has nylon sleeve bearings. That and better quality materials together make more durable product.

Paying for high end gets you carbon fiber parts which mainly save weight and ceramic ball bearings which are mostly useless in bikes.