Reading the documentation
[source], it appears that ATA DMA is disabled to increase compatibility. Is this information accurate?
I believe so, aside from a faster bootup, having ATA DMA wouldn't do anything for you. Once booted, nothing should be writing to "disk" other than an occasional configuration update. Even during boot, I wouldn't think it'd make all that much of a difference since the "OS" is so small that there's not that much to transfer.
Topic: ATA DMA