Its like USB2.0 which can make up to 480 Mbit/s but in real life you will get only about 320 Mbit/s or less even on modern PCs which makes it slower than FireWire 400.
IDE on amiga is a mess, it eats all your cpu. Only the Deneb has DMA which makes it useable with IDE devices without freezing the whole system during file operation. If Deneb is too expensive for someone I would rather go for a Z-III SCSI controller.