Besides that, theoretically, the dual-channel mode doubles the bandwidth between the CPU and the memory, it does not mandatorily result in a perceptible impact on the performance of a specific program. In most of the games we tested, there was not significant performance gain when we enabled the dual-channel mode.
In some cases, there was actually a performance drop when the dual-channel mode was enabled. It can be a simple statistic fluctuation (the ideal was to repeat each test dozens of times in each configuration to avoid it, which is not viable) or even a behavior of the CPU memory controller, working more optimally (maybe with smaller latencies) in the single channel mode.
The fact is that, in the configuration we used an in the games we select, there was no advantage in using memories on dual-channel. This means that, in some cases, when an 8 GiB module is less expensive than two 4 GiB ones (for example), you can install only one memory module with no fear of losing performance.
But please keep in mind that those results apply only for the hardware and software configuration we used. With a different setup, results may be different.