Make wifi TX rate in menu bar instead of wifi icon
While it won't display the TX rate permanently holding down the option key when you click on the WiFi pizza slice icon will display a great deal of information about your current WiFi connection, including the TX rate, RSSI (Received Signal Strength Indicator) and noise levels. 

The RSSI and noise levels are more useful than the TX rate in determining the strength and health of your WiFi connection.