It seems that when you said I can't see much difference in read and write, I managed to read this as I can see..., which prompted my response to look at query plans. (Hm, I wonder what Dan will have to say about me missing a not also when reading...) Sorry for the confusion.
I think one thing that would be very interesting to look at is the wait statistics. Before you start a load test, run this:
DBCC SQLPERF ("sys.dm_os_wait_stats", CLEAR)
When the test has completed, run
SELECT TOP 20 * FROM sys.dm_os_wait_stats ORDER BY wait_time_ms DESC
You may have to filter out benign waits that are unrelated to the tests, but we'll see.