Load testing BizTalk is not an easy task. BizTalk Benchmark Wizard is an open source tool that checks the performance of your BizTalk Server installation using pre-existing scenarios and validates it against some known results. This will help you in determining if you are getting the most out of your BizTalk Server. It is simple to install. It creates Hosts and Host Instances on the servers in your BizTalk Server group. It generates test data, runs the tests and publishes a report of the results.
You can learn more about it here