Well, what kind of performance tuning are you doing ;-) ?
Did I get you right? You are setting properties within your build file and you expect to read them (as system property?) in your testcase? This won't work, since ant uses "private" properties for every ant project.
If ant task you are using for tests doesn't support to pass properties as arguments, write your own task as wrapper and pass them.
Could you email me so that I can ask a few more questions? What you said sounds complicated. My simple mind needs more help.