Does Nimbi Burst boost break efficiency?

With an 18-second cooldown on a 20-second buff, you can keep the bonus running permanently. Recast it on cooldown and the team never loses it. The same cast also raises team damage by 15% for the same 20 seconds, so you are not trading damage for break.

Nimbi Burst is Nimbi's core skill, and it is also one of Turbo's attacks. The name also turns up as Nimbus Burst. It is the same skill and the same buff.

Doubling it to 30% with Turbo

On Turbo, the buff doubles. Cast Nimbi Burst while Turbo is in Shrouded in Mist and party break efficiency rises to 30%, with the damage buff going to 30% alongside it. Shrouded in Mist triggers at four stacks, and Turbo builds those stacks passively while off the field, reaching the state after 20 seconds.

In practice that sets the order: let Shrouded in Mist come up, then cast Nimbi Burst. A cast made before the state is active only gives the base 15%.

Not settled: How Shrouded in Mist activates is not settled. It is given both as triggering after four basic attacks and as triggering when a meter filled by dealing damage is full. That decides how reliably you can get the 30% break efficiency version of Nimbi Burst rather than the 15% one.