A further update. I had reported earlier that my Nec 2500 worked fine on the SiI 680 controller, but I just decided a short while ago to swap out the 2500 for my new Benq 1640 (have to play with the new toy, don't you know?). The Benq 1640 does NOT play well with the SiI 680, so I searched for this thread and reread the comments, especially where Dee said that Liteon and LG drives worked well with the SiI 680. With that in mind, I swapped drives around and connected the Liteon 1693 to the SiI 680, and SUCESS!
Previously, when I stated that my Nec 2500 worked well with the SiI 680, I might have been misleading others unintentionally. The Nec 2500 only writes at 8x max, and I never used it to read, so I don't know whether it would read at greater than 8x when attached to the SiI 680 card. The limitation I found when trying to run the Benq 1640 on the SiI 680 was that it would neither read nor write at faster than 10x (looked like the transfer rate test posted further up in this test. So it is quite possible that the Nec 2500 was limited when connected to the SiI 680, and I just flat wasn't aware of it.
My apologies if I mislead anyone.