I have an article on DevX that compares SQL Server 2008 Reporting Services performance to SSRS in SQL Server 2005, from an execution perspective as well as development.
As it is currently in CTP, these things may change.
Link to the article:
SQL Server 2008 (Katmai) Reporting Services Performance: Two Steps Forward, One Step Back?
http://www.devx.com/dbzone/Article/37653