Old zsnes used FastRom speeds in all areas [Registers, Main Ram, Low Rom]? That might make it noticeably faster than any "FastRom" conversion.
Don't think entire rom has to be $80+ converted - just important game logic that always gets run every frame or two in-game.
A large, fast Sram can be used in place of main Ram for a speed kick.
Wondering though.. because I'd imagine a lot of snes tech experts probably already tried a Gradius III / Super R-Type FastRom approach (using intelligent logging, disassembler tools) and maybe got barely appreciable results.
And there's the curiosity whether they spin idled anytime they missed a vblank frame. 60 -> 30 -> 15. That port skips a couple frames at times. Maybe intentional though - "bullet time", "reaction time". PS2 Gradius III can be wicked hard at full speed.