Basically your turning Off DMA transfers. Very good point sirtoby, I think the fastest PIO mode is PIO mode4 with something like 16.6MBs transfer rates. If you think some of your games levels are loading slow now, what will it be like without DMA. Besides the slower transfers, I would think HD CPU usage would go up too, because the DMA controller is offloading work from the CPU when enabled for HD transfers.
You have to ask yourself if the tradeoffs are worth it?
*Nothing wrong with playing around with it and seeing for yourself though. Have some fun!