Grenade

They don't stack?

Or is it just impractical because of grenade's pomegranaty range?

Using Flamethrower with maxed Elemental Boost with Cannon is a net gain of theoretically ~50%/s. Using Grenade with Cannon is theoretically a net loss of ~124%/s

Using Flamethrower with maxed Elemental Boost with Torpedo is a theoretical gain of 126%/s per target. Using Grenade with Torpedo is a theoretical gain of 27%/s per target.

However, Grenade cannot fire at standard speed and hit more than one target in any bossing scenario, if any targets at all. If it were a full screen AoE that required no charging, it would still be very marginal of helpfulness, and only on bosses that did not resist fire.