There is a thread here http://old.nabble.com/Tomcat6.0.16-is-too-easily-to-be-OutOfMemory-compared-with-Tomcat5.5-td16921744.html
added 1 virtual user per 10 secs.
Only 30 virtual users, OutOfMemory was occured at Tomcat6.0.16,
but Tomcat 5.5.26 was 270 virtual users!
So is it risky to upgrade from Tomcat 5 to Tomcat 6 as It doesn't seem obvious to configure Tomcat 6 to get the same performance as Tomcat 5 ?
In fact in the thread above nobody was capable of knowing the solution.