Will DMA still be an issue if the drive is seen as a scsi drive via ide-scsi (sorry, now gone past the limits of my knowledge here)? Does scsi use DMA?
SCSI doesn't use DMA, but the IDE interface bit of ide-scsi should. I'm not quite sure how this works in the real world, because how do you enable DMA on an IDE drive that doesn't exist.
BTW, while we're on the topic, do you know what the data transfer speed for CD reading is? I've read that 1x DVD reading/writing is several times that of 1x CD data transfer but never seen the figures.
CD 1x is 0.15 MB/s
However the maximum rated speed of a drive is just that, you would be unlikely to obtain a sustained transfer rate of 7.8 MB/s from a 52x drive.
also
Most drives have adaptive speed controls, if the error rate is too high because of an imperfect disk then they slow down, even if the drive doesn't actually slow down then I think the error checking has the ability to request a re-read of a track, thus slowing down the output.