If you use one of the "unclean" (i.e., non-overlapping) channels, then you multiply, but at least a factor of two, the amount of other APs that will interfere with you, and that you will interfere with.
A further potential effect is that another system may see your interference and decide you are a "rogue" AP and begin mitigation practices, that essentially shut down your communication.
Part of the design challenge is to stay on the three non-overlapping channels, and space then APs/Antennas such that they overlap enough to give smooth roam transitions, yet maximize the coverage.
The two easiest ways to accomplish that is to control the transmit power, and use a variety of antennas of different types to efficiently shape the coverage.
That might mean that you'll need more APs, but for optimal coverage & service (in this band) that is the only way to go.
As always, a complete, comprehensive site survey is MANDATORY ... or you're just wasting your time & money (or your client's time & money) playing trial & error.
Good Luck
Scott
