If nothing else, they will be more active until all of the cardinals are eaten!
Seriously though, in the last 16 months, we bought well over 70 jumbo cardinals before giving up. A few would die off on their own in QT, but usually they died due to discus attacks. For some reason, the last 7 have been in the tank for over 6 months. Our suspicion is that when we added the endlers to that tank, the discus had easier prey whenever baby endlers "showed up".
Other people have had great luck with cardinals in their discus tanks.