OK so I did a bit of research, and ran into many empty threads...
just like my own
But I decided to give achi mode a try as per some suggestions I found. I had to do a little registry tweak to get it to work in vista, but I had it up and running.
HD tune was also reporting the drives as running in UDMA mode 6
So I did some testing, on both of my drives (old hitachi 160gb and caviar black 1tb).
Turns out all achi did for me was increase the CPU usage by about 10%, and it seemed to cap the burst rate at 100MB/sec. I also found a couple threads about this, but they were also empty.
Anyways, I went back to the regular IDE emulation (is that what it is?).