I made one of these shotguns to try out at rank 6, it's pretty beastly. I added a +2 mod slot augment to it, since 4 mod slots is nice, even on a shotgun. The base pirahna shotgun doesn't have the huge damage per shot that a few others might have, but it's got high rate of fire, big clip size, and a lot of extra ammo so you don't always have to depend on the health to ammo conversion augment. Keeping health topped off just through normal Incinerate (with Life Support) use works well. I did Drack's loyalty mission right after making the shotgun built, and it just completely destroyed mech's and other armored enemies.
The Piranha isn't bad. I keep one around for some missions as well. I use the Dhan primarily though. It doesn't hold many shots and you have to be really close to ensure you don't miss, but it fucks shit up fast. I've got it at level IX and its damage sits around 1535 or something right now. I'm using it with the mod that adds 25% shields after you empty the weapon. That keeps your shields topped off while you reload. Damage bonuses from tactical cloak and in the skill tree make it the hardest hitting weapon in the game as far as I know.