Airport Extreme to Extent Airport Express

There is a lot of discussion about using the Express to extend the Extreme. But I'm wondering if there is a major disadvantage to using the Extreme to extend the Express. My airplay speakers are right near the router, so I want to keep the Express there, which means it would have to stay connected to the router and function as the base. But I bought an Extreme to enhance the power of our home wireless system, both in terms of range and speed. Will speed decline if Extreme operates as an extender? Anything else I should be aware of?