Just bought an Airprort Extreme to boost my WiFi performance in my house. Everyone tells me its the absolute tops for performance in n or ac. I have tried several locations but cannot get more than 180Mbs on my iMac when is on a dedicated 5GHz channel on ac. iMac is no more than 15 feet away at the top of the stairs from where the Extreme is so no walls or ceilings as such. Pretty poor by any standards. Interference is around -72 so not that high. I am sending it back to Amazon for exchange in case there is something wrong with it but it seems odd for such a highly rated device to perform so badly. Anyone else had similar experiences or what should I expect given the location of the devices ?