[LAU] rtirq - what does it improve, and how can I measure it?

I understand that the rtirq script is designed to raise rtprio for audio devices, which is made possible on the vanilla kernel since 2.6.39(?), if passing the threadirqs option to the kernel at boot, and having built it with the CONFIG_FORCE_THREADIRQ. From my experience, I have not had any performance boost using the rtirq script, but I have read about it helping those who are getting xruns due to irq sharing. So, I'm wondering. What picture do others have of the benefit of the rtirq script? ..and are there other ways to measure improvement for audio operation other than spotting xruns at different latency settings, and reading the rtprio for various devices using the 'ps' command?