I use an AirPort Express to extend my Wi-Fi network and it works great. My question is whether I would notice a significant signal boost by plugging in an ethernet cable into the AirPort Express rather than accessing it over a Wi-Fi network?
I would not expect a significant Signal Boost from Cabling the connection back to the main network. However, if you have several devices connected to the AP, then you may see an increase in speed. Say you have three PCs connected to the AP at 54Mbs, all trying to open large files from the file server. Right now they all share the 54Mbs connection to the wired network. The WiFi connection to the network becomes the bottleneck. If you replace it with a 100Mbs or GigaBit cabled connection, then the PC's will down the files faster. Of course this could push the bottleneck somewhere else, like the internet connection if they are downloading music or something...