Indeed. If you have a look at my results with Verbatim and CMC, it is clear that lower speeds does not translate into lower BLER. Jitter does tend to decrease at lower speeds, but look at the 4x CMC burn - it is higher than at 24x.
Of course, a lot depends on the drive/firmware and the media. The situation nowadays is that both are optimized for higher speeds. I found firsthand that 4x burns are generally inferior to those done at, say, 16x on the Premium.The only exceptions was MFSL's Ultradiscs, rated at 12x. Such discs are rare nowadays, and at least BLER-wise, offer no advantage over common top-grade media. I wish I had done jitter tests with those gold CD-Rs to compare to Taiyo Yudens but I did not keep the test discs and don't feel like throwing another $20 on what I feel grossly overpriced media.
Plextor Premium, like probably all drives on the market in the past few years, does not support speeds below 4x. It is a hardware limitation and as such cannot be worked around.
CD-DA stands for Compact Disc Digital Audio, i.e. audio CD.