I did a quick test using the updated diskspeed program from Joerg Strohmayer:
AmigaOS 4.1 Classic, Deneb PIO Mode: 3.55 MB/sec, 96% cpu usage
AmigaOS 3.9 BB4, Deneb DMA Mode: 3.866 MB/sec, 41% cpu usage
So you are correct with hard disk transfers - DMA does make a larger difference.
Same disk in the same machine with the same partitions under AmigaOS 4.1 Classic and AmigaOS 3.9, using FFS2 (DOS\07).
I was 28% CPU but meh, not a big deal oh and around 6MB a sec but been a while since i re-tested. That was a Z3 Fastlane with Segate 540 MB SCSI2