I'm not convinced that the -500s are much better than the SLx series. While they may have spent a lot of time in the US, they spent most of their time away from RE. I've had as many delays on the -500s as anything else in the fleet. I do seem to get lucky on the 201s though, no delays at all!
Frankly, I'm not sure the aircraft are really to blame, although they do seem to have their share of technical delays. Having one or two hot spares in the network seems to have improved things - the 3-4+ hour delays seem not to happen as much. I see they have had a Titan 146 in during the week to cover. The real cause of delays is probably more closely related to 8 sector days with 20-30 minute turns, there is just no time to recover even 10-15 minute ATC delays and they really build throughout the day if that's the way it goes.