I think you are comparing apples with oranges. Sure under certain circumstances, mySQL can perform better. The advatage though of systems like MS SQL, Oracle and DB2 is that they are actually scalable. Frankly for a little forum DB, mySQL is generally fine.
I do not put any weight into benchmark test from e-week, who I think are just a bunch of laid off Admins, results I trust are i.e. tpc.org, oh wait mySQL does not participate, but yet they provide their own results.
To put this to an end, I am sure mySQL would be a good db for a forum in general and it may even outperform msSQL. Thus for me personally there are some downfalls to be using it for my application development, mostly because of the mySQL Dialect being inferior to T-SQL.