No. Voidwalkers and Sunsingers are balanced, they need to sacrifice somethings to improve others, so while Nova Bombs and Solar Grenades might seem OP to it's victims, those warlocks had to sacrifice some other good perk(s) to get them that good. (Ex: Solar Grenade becomes strong by sacrificing your extra grenade and making you super last half of the time). Stormcaller is actually OP already, the super lasts the same time wether you kill something or not. You can kill an entire enemy team and wait for them to respawn and kill half of them again in a single super. The only change i would make is, while pressing trigger during Stormtrance your super is consumed faster. If your warlocks feel underpowered, it's because you are using the wrong perks in the skill tree.