We are using .Net remoting now between Web Server and Application Server farms. I am consider to use WCF for new modules.
Good news: The WCF and .NET Remoting are really comparable in performance. and according to MSDN article A Performance Comparison of Windows Communication Foundation (WCF) with Existing Distributed Communication Technologies WCF even approximately 25% faster.
I hope to extend this post when I will collect more info.