If I'm reading these DPS charts right, Impact Slider has practically the same DPS as Steady Shot if you factor in the roll and Standing Snipe. A roll cancelled Impact Slider is 120 frames, and Steady Shot is 117. Both output around the same damage per use as mentioned. Thing is, Impact Slider fires in a compressed burst when compared to Steady Shot. That means you need to secure the kill with Steady Shot during the windup of a competitor's Impact Slider to have higher DPS. Meaning the enemy has to just barely not die from the last round of fire. Otherwise Impact Slider has like 10% or more DPS on Steady Shot due to not factoring the roll into the actual kill shot. And that's not factoring in that Impact Slider's DPS isn't lowered as much by having to dodge due to iframes and already counting a roll in.
So Steady Shot is only better in three scenarios. In the first, you need to be able to reset Standing Snipe by staying still for the last few shots of every Steady Shot, you need to never roll, and you need to be fighting enemies that barely survive n-1 Impact Sliders. In the second, you need to be fighting an enemy that requires Steady Shot's insane PP efficiency. In the third, you're hosing down trash mobs where Impact Slider would overkill maybe one or two.
On the topic of Slideburst-0, it has the bonus of activating Tech Arts skills and Standing Snipe if you use it to dodge. Impact Slider doesn't do as good of a job on the former, and I haven't tested the latter. Plus Slideburst-0 lets you pull out Ra's actual highest small target DPS PA easier: aerial 2-tick Piercing Round. It's nearly double the effective DPS of the other two. You also look awesome dancing around the enemy while sniping it.
Edit: to correct an earlier post: Piercing Round does the same damage per tick on double/triple hits if you space it right. You're probably landing the later hits in a different hitbox.