The bottleneck is not the SATA bus but the harddrive. Depending on your harddrive, you may not be able to burn more than one disc at a time without killing the harddrive performance, or you may be able to burn 2, 3, or more.
I have two different SATA harddrives in a 4-5 year old system (one harddrive is newer than the other).
One harddrive (oldest) can only handle one simultanous burn to DVD - if I attempt two, the combined burn time is much longer than if I simply burn two DVDs after one another.
The other harddrive (newer) easily supports 3 simultaneous burns at 8x (my normal burning speed) and will often handle 4 with no or only a couple of buffer underruns.
There is no other performance indicator that would indicate one harddrive is significantly faster than the other, but one handles multi-tasking easily and the other handles it badly.
You can try reducing your burn speed - 8x is not that much slower than 16x, since 8x is Z-CLV or P-CAV and takes around 8 minutes, while 16x is usually CAV and takes 5Â½-6 minutes.
It's possible that your harddrive simply can't cope with the multi-tasking.