The latest version is veeery slow in IE7 when scrolling, and can prevent it from responding.
Not sure if something in particular triggers this or if it's just the counterpart of recent enhancements, but it is not usable as is anymore.
This is a critical issue.
I guess you applied the polyfill the Paul Irish way, meaning like:
That's the first thing you probably should not do. Reason being that you create a dependancy chain accross the DOM, where child elements recalculate based on the size of the parent elment, which is also calculating its own size and so on and so forth.
Try to apply the polyfill just to the elements where you really only need border-box.
Apart from that: Can you show me an example-page, please?