I am wondering how do you stop people who are using IE 8 from going to Compatibility mode?
Adding a tag to your page will not control the UI in the Internet Control Panel (the dialog that appears when you selection Tools -> Options). If you're looking at your homepage which could be google.com, msn.com, about:blank or example.com, the Internet Control Panel has no way of knowing what the contents of your page may be, and it will not download it in the background.
Have a look at this document on MSDN which discussed compatibility mode and how to turn it off for your site.