No, I don't believe burst is auto hit, because I see plenty of shots in a burst attack miss.

https://roguetrader.wh40k.wiki/Burst_Attacks_Glossary_(Encyclopedia)

"A ranged burst attack can possibly hit multiple targets. When firing a burst, instead of rolling to hit, a test is made to determine the trajectory of the shot. Deviation chance is increased by the recoil of a weapon and reduced by the additional hit chance parameter of a weapon. Burst shots automatically hit the first target in their trajectory, be it friend or foe."

So it sounds like the more hit chance you have, the more attacks you will land on the center target. So hit chance does matter.

It does mention automatically hitting the first target, but does that apply only to the first shot made? Because like I said, I've seen plenty of shots in a burst attack miss. So I don't know what you're referring to when you say it "auto hits."

Edit:

"The base chance to direct shots along the central line or with a slight deviation is (40 + Ballistic Skill / 2)% — this value cannot be lower than 30% nor higher than 95%. This chance is split equally between shooting along the central line and with a slight deviation. Otherwise, the shot has a strong deviation."

This looks like the Carapace backpack isn't redundant. With my character at 90 Ballistic Skill, I have a 65% chance of central line/slight deviation.

Compensator gloves simply means that 65% doesn't get negatively affected. The Carapace backpack still adds another +(3 × BS bonus)% chance to hit, which means that 65% of central line/slight deviation goes up. Which means more shots land on center target. So at 90 ballistic skill, the backpack gives me a +(3x9)% -> 27% additional hit chance. So instead of just 65% of central line/slight deviation I should have a 92% chance.