We used PerformanceTest 6.1 by PassMark which you can find out more information about here. It has a handy Advanced Networking section which is perfect for our testing.
Doing our best to emulate a real-world performance scenario, I setup a server running Windows 2008 R2 (x64) Server, (2x Xeon Quad Core 3.0 GHz 1333MHz FSB, 4GB DDR2 FB-DIMMS, 2x 146GB SAS drives in RAID 1) and the client PC was an MSI Wind 200 With built-in Intel 802.11n (Intel WiFi Link 5100 AGN) adapter. The results were gathered by sending data from the MSI netbook to the server at different distances with the built in adapter, a TPLink TL-WN821N and then a TRENDnet TEW-644UB Dual Band Adapter so we could test both 2.4GHz and 5GHz. Average transmission speeds were recorded for each.
We had some interesting results here. With the 2.4GHz band we saw the usual, between 64 and 150Mbps connections. With the 5GHz band we saw all the way down to 20Mbps. We also did not see anything above 81! - This began to worry us, so we tinkered around with some things in the setup for that band. What we found was using the default channel of 36 was a bad idea. It never got a decent connection and transfer speeds were in the single digits.
In the end we found that channel 153 (5.765GHz for us) was the best for connection and speed. With this channel going our 5GHz band stayed between 81-150Mbps and speeds were excellent, as you will see below.
*signal travelling through wet wall and main house electrical panel
Looking at the speeds here, the TEW-673GRU does not do too badly at all. Even when penetrating multiple walls and walls with electrical lines in them, we maintain good speeds on both bands. For laughs we ran some MKV files over each of the bands from the same NAS source (and in the same folder) and we had smooth playback on both. We also tried a large file move over the 2.4GHz band (again to the same NAS source) while watching the same HD MKV file on the 5GHz band and things were still quite stable and clean. I was very impressed.