I would assume that without game limitations, sunsingers would be able to revive themselves, toughen their armor (radiant skin), and utilize light boosting (song of flame) all at once, possibly more frequently. Other than that, melee's probably wouldn't have a cooldown in an actual encounter, so stormcallers would essentially be able to arc electricity at any given moment, making the insanely powerful for however long their light can hold it. As for an explanation how? Not entirely sure, everything I've written here is all speculation, but my guess is that they're abilities are as strong as their light, and that would be the determining factor for how often and frequently they can use their abilities without tiring themselves out..