The other program I used was AJA KONA SYSTEMS TEST. I mounted a drive from my mini, set the prefs in Kona to allow network drives, then have it generate a 1GB file. Below is a test I just did. 10.4MB/s = 83.2Mb/s, which isn't bad. Almost as fast as wired 100Mbps connection. (But not faster than the 100Mbps ports on the back of the router).
Have you seen the status screen in the AirPort Utility that shows you how fast each of the wireless clients are connected? That's pretty cool.