Steve, I wonder if you don't have too many tempdb files. I have lost track of the number of clients I have come across that cause IO performance problems by having too many files (of any SQL Server type) on too few spindles. Causes head thrashing and you get killed by the disk access/seek times. Unless you have demonstrated tempdb object allocation issues or have a boatload of spindles (or SSDs) you may get better IO with fewer files.
Best, Kevin G. Boles SQL Server Consultant SQL MVP 2007-2012 TheSQLGuru on googles mail service