Not sure how sound that logic is, first of all this is not really what iRules and the F5 devices are designed to do they are not designed to stress test your web servers. You may be able to do this using a monitor or an iRule but I am not sure I would fully trust the results. Second you are putting the F5 in front of your servers so you would want to test through the F5 not from it to get a proper stress test of the infrastructure. Then run the same tests directly against your servers to test the individual load levels so you can determine what setting you may want to use on the F5 to optimize the performance of the application.