Standard PC floppy drives can run at variable spindle rates. The problem is the drives vary in quality and some can't reliably do it (TEACs and naturally Sony were the best at this). The Central Point Copy II PC Deluxe Option Board was an add-on disk controller that could read and write 400k and 800k GCR formatted disks using a standard PC drive. The Amiga could also read/write the GCR disks using an add-on called "Sybil" and used a trick that altered the clock speed of the "Paula" chip that controlled the floppy drives. Amiga double density floppy drives were lightly modified PC drives for the most part (the only platform I've seen with disk insert notification AND manual eject!).