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.

True, but the range and button mashing aspects still apply.

Pretty sure all mages have a fixed faster(2) speed. But i guess i wouldn't know.

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.