Here is a good resource about IE Compatibility:
http://webdesign.about.com/od/internetexplorer/qt/force-compatibility-view-in-ie.htm
Also you can force your customers to install a plugin for IE to solve the issues Microsoft's Team did not.
Google Chrome Frame:
http://www.google.com/chromeframe. Of course Microsoft advices not install for security reasons.
But no word from Microsoft about the speed with Google Chrome Frame installed:
http://www.computerworld.com/s/article/9138459/IE8_runs_10_times_faster_with_Google_plug_in
Sooner or later X-Cart should be completely HTML5 + CSS3. doctype will be only html (<!DOCTYPE HTML>)