Using Airport Express as a wireless repeater - Significant drop in wireless speed
Any time you try repeat a wireless signal you'll sacrifice data speeds. 

This "feature" isn't highly pushed on the Airports because of this. I've seen no consumer wifi routers/repeaters that do this well. 

Your best bet is to run an Ethernet cable and setup the airport express as a second access point. Use the same SSID and password as your Extreme and your devices will pick the stronger signal.