Update on my previous Wi-fi woes: I found an old Linksys WUSB54G USB Wi-fi adapter laying in a drawer and plugged it into the studio Windows 7 box instead of the TP-Link PCI card. Win 7 found and loaded the driver automatically, and DPC Latency checker now reports a maximum latency of 331 microseconds. That’s not quite as good as the 74 microseconds with no Wi-fi, but a whole hell of a lot better than the 6000+ of the TP-Link card. Not only that, the Wi-fi signal is has gone from Poor with the TP-Link card to Good with the Linksys USB adapter..
So my conclusion is: TP-Link products appear to be cheap for a reason.