This might be a little nit-picky, so I apologise in advance, but I understood the DBCC DROPCLEANBUFFERS command to only clear clean buffers from the buffer pool. If you don't use the CHECKPOINT command before calling DBCC DROPCLEANBUFFERS, you haven't forced dirty pages out of the page cache onto disk to maximise the effectiveness of DBCC DROPCLEANBUFFERS. (See http://msdn.microsoft.com/en-us/library/ms187762.aspx)

If I'm right, this may also cause misleading performance results (or at least difficult-to-repeat performance results) between executions.