Already explained the power issue, if the radios are too close or too loud, they can overdrive the receivers and cause performance issues. If the intent is to get max performance in a lab, you likely won't. They aren't designed for that and don't work optimally when the APs are too close together, too loud, combination of, etc. Max rate is 120-140Mbps, so you getting 100Mbps is likely max for 11n HT40 in that setup.