I’d like to expose certain methods of my service layer as a WCF service. If I make my whole service layer as wcf methods, would the calls from my web app be slowed down because I’m converting to and from xml for no reason? As shown in this image:

How would my project be layered or structured?
WCF services are just a contract.
You can easily expose service classes as a contract for a WCF service and also just new them up like normal classes for use in your MVC application. You won’t be creating XML or SOAP/Http messages if you simply use your service classes as just classes.