I managed to clear with a vertical fourth photo and only got 19% recharge on it, so maybe it depends on the individual bullet density?
Where are the likeliest improvements?
Welp, it might be hard to do with a shot that early then? Not sure to be honest.
I've been trying to step it up on 10-1 and save another 1-2 second by starting with the new strat from the second photo, got withtin 9x% of the last shot a few times, definitely doable, most likely doable within 100 tries or so.
10-2 while recently improved can surely be about a second faster still if you stay closer to Iku.
12-5 can still be improved some by being faster, but so can probably 90% of the scenes

Some small improvements shouldn't be too hard on 4-5 and 5-4, there's better strats for both of those, I can show what I mean if you want.
And there are big timesavers on many of the hard-to-speedrun scenes, such as 11-2 and EX-7 but they are also the most difficult/unlikely ones to pull off I think.