For this I’ll use Impact, Battlefront, Steady, Onslaught and Do or Die, and a run of the mill Torgue pistol with 100% splash.
Without COMs the current bonus is +80% gun damage and +65% grenade damage. This results in:
1.8 + (1.8 * 1.65) = 4.77
Leg. Engineer adds 5 to Impact and Battlefront giving +50% gun damage and +30% grenade damage, resulting in:
2.3 + (2.3 * 1.95) = 6.785
An Impact Grenadier with 6 to Impact and 5 to Steady gives +24% gun damage and up to +72% grenade damage, which results in:
2.04 + (2.04 * 2.37) = 6.8748
Finally, an Expert Grenadier with 6 in Steady and 5 in Impact gives up to 20% gun damage and +77% grenade damage, resulting in:
2 + (2 * 2.42) = 6.84
So on a bodyshot and with Onslaught active, both Grenadiers will outdamage the Engie. However, the Engie’s higher gun damage will let it do more on a crit, and it will also do more if Onslaught is not active. I still use Engie anyway though for reasons stated in my own build guide.
( I don’t keep either a Flakker or Kerblaster on any of my characters)