Regarding ATT Speed and Inner Ability

I've got a couple of points I'd like some clarification on if anyone could provide them.

1. Lately, in YT videos, I've seen an increasing number of Dual Bladers that case decent SI and also use a MPE att booster potion for their videos.

I've always thought that speed was capped when using a booster on dual blade, but have I been mistaken?

2. Regarding inner ability, how hard is it to get, as a unique rank, 35%+ buff duration as one of the lines? I've not Azwann'ed since the revamp, and I've got no idea what to expect.

I currently have 3-line Unique potential, would it be worth using CS circulators to move it up to legendary and go for %buff, or should I just use the Conquerors coins to buy up circulators?

Thanks in advance if anyone could answer my question

Also somewhat unrelated, could anyone tell me the range calculations for DB?

or, how much 1 ATT would be equivalent to? I do vaguely remember the old formula where you divided total stat by total att, but is that still valid?