Yeah, 15/20/30hz do feel quite similar, and seem to get similar kill times on gems. (shooting them until they disintegrate entirely).
It wouldn't surprise me if 30hz is valuable, though, given the effort TZW_Guile and TYR-Yeti have described in getting it working perfectly for this port.
Really interesting thread by TYR here, on his experiences with analogue and synced autofire on the System 16 board over the years, and the tuning of the ACA version.
Apparently, the recent Astro City Mini version doesn't emulate the board's slowdown accurately. So some were surprised when the ACA version's BGM tempo dropped during rapid fire, just like it will on the PCB. Hamada mentioned this on the Arcade Archiver podcast, it seems.
TZW's comment.
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For my humble part, I'd forgotten the game's odd two-button charge input for Level 2 magic. Makes Cotton 2's FTG commands seem less unexpected, haha. For my initial dabblings this evening, I settled on three buttons, ABC layout. [manual shot] [manual bomb] and [30hz shot+bomb]
This way I have the Charge Attack (hold bomb) and Homing/Barrier (hold both with appropriate gem) ready to go, while 30hz shot+bomb takes care of general crowd control. L2 Magic won't trigger if you hold down rapid shot + rapid bomb buttons, even at matching frequencies; I guess the game waits a few frames to register, with that very handy audio cue.
"Baiyahh~!"
With the item juggling, there are times it really pays to shoot without bombing, or vice-versa! I'd forgotten how aggressive your Option Faries' rolling bombs are; anything on the ground is getting hammered. Feels rad.

Also, I find the manual tapping is quite comfy in this game, with generously fierce output. So I was doing quite a bit of that, too, especially when sniping gems for just the right colour.
Overall, I'm pleasantly reminded of just how articulately this plays, between the charge attacks, juggling, and tricky enemy design/placement. The Homing in particular almost feels like an IREMesque Force Device, at times, and the Barrier's body-ram is life-affirmingly tactile, just like TYR recommended.

Great game I can appreciate more now than I did back then.
^^^ (X)68000 default hiscore ala TFIII, daww
