Is it possible to hit max damage on each line of uncharged big bang?

I don't think that's the case. With spamming BB, you can probably do about 120+ casts per minute (about 2 per second), and each is 5 hits so ~600 hits per minute. While with CL you do ~88 casts per minute, and each is 6 hits so that's 528 hits per minute. That's about 70 extra hits.

Also I wonder if Decent SI helps uncharged BB when it's spammed.

Nope. Decent SI does noticeably affect mages.
It's different from W.Att based classes in that it changes casting speed instead of weapon speed.

No, Magicians use Weapon Speed like every other character in the game. There is no difference from "Casting Speed" and "Weapon Speed" other than the flavour text of it being associated with Magic. It is capped at 2. Big Bang is the same speed as Paralyze. Stop misinforming people.