In which case, I will continue to go with what I've seen. Single channel will slow things down by probably at least 5%, though more is likely depending on the task. If the task is memory sensitive, then you'll see more like 10-20% less speed. For gaming, your probably looking at 10% on average, just like my link showed.
You have to remember that while doing X task, there is more then just memory read/writes. There is a lot of CPU time involved, and transfers to/from the harddrive and video card. Just because you slow the memory (or link another harddrive in an AID0 array) doesn't mean your scores for something will change. Single channel WILL slow your performance, but it won't cut it in half, it probably won't slow you down by even 25%.