If the beam range is annoyingly short, swap Pistol Elem or Galv Shot for [Ruinous Extension].
I opted for [Lethal Momentum] because the damage falloff is quite bad, and projectile speed improves it. Without it, there's not much point of a longer beam since the damage will be bad at longer ranges.

If you wanna try a grenade spam build, swap Pistol Elem for [Synth Charge].
Idea being that you swap weapons after the grenades go out but before the reload actually fills the magazine. Wait a second before swapping back to the [Catabolyst] and you'll have a little bit reloaded. Empty it out and you can throw grenades again. Repeat.
I think Galv Shot is multiplicative on this weapon but only if the grenade is a direct hit. So ye try to do that on tougher targets