Does 4.1 load in IE using Chromeframe? Chromeframe is blocked on my corp machine with IE8 so I can't test. But, if it works I might lobby to IT to open it up as a permitted add-on.
I looked at what qualifies Ext.isChrome and if that's what is used to determine what is rendered then it looked like it wouldn't catch from the useragent that chromeframe was working in IE. But, perhaps that has nothing to do with how the dom is laid out?