Note that the compat mode may mean that you are miscounting IE8 users in IE7 compat mode as IE7 if you are doing a naive user-agent detection (I only mention it because I did and started blocking IE8 users!)

Both will emit "MSIE 7.0" in the user-agent, but IE8 will have Trident/4 while IE7 lacks any such token. MSIE 11 does away with "MSIE" in the user-agent altogether.

There's a X-UA-Compatible header (and meta variable) you can emit to ask for no compat checks.