Even if your DVD will be read at lower speeds, by default, it is recommended to KPROBE at 4x with ECC 8. GOOD media should give good readings even at 4x scan. Try other brands until you find the right one. I've had consistently high PI/POs on my 811, until I found the right media to use and now my scores are within spec using 4x ECC8 scans.
As to DVD Video, it is read at 1x by your standalone DVD Player. Sure you can KPROBE your disc at 1x, but it's unlikely you will want to spend 2 hours waiting for the scan to finish. You will likely also get excellent results, that is what counts for your DVD Player since it will certainly not read it at more than 1x.
However, good media combination for your writer should produce good result even at 4x.
Another note, if you people use bitsetting to DVD-ROM your PI/POs will be higher.