I never noticed a difference on various setups. There are really only three fundamental things that matter on a rear derailleur... the pulleys which have more to do with overall drivetrain efficiency, the springs (weaker springs can be problematic with narrower cassettes, and it would be the same using an indexed bar-end as it would be using a Campy Ergo, although a friction setup would give you more control over how much pressure is applied per shift), and then there's the geometry (so shifters need to match RD for indexed gears, again moot with friction).
So no, I don't think it's really a matter of derailleur quality with modern derailleurs, just a matter of derailleurs matching the shift mechanism. Things just need to be matched up correctly, unless you are in friction mode.
Personally, I run a pair of Campagnolo Athena 11 speed shifters that I got on sale on Amazon for $100 a few months ago... they work great with a Shimano LX RD and Shimano 9 speed drive train. You can get 10speed Veloce shifters if you order from Ribble for right around $100 and those will work great with Shimano 8 (or with Shimano 9, with an alternative cable routing style that is commonly referred to as "hub bub cable routing."
*edit to add* agree with brad, good cables are key as well as properly run cable, especially with 10 and 11 speeds.