I think "compression" is the wrong term to use when describing the sound of MBHOs at high SPLs. I think the most accurate way to describe it would be midrange smear, the image becomes distorted and the midrange detail goes out the window. It tends to make the mics sound farther from the source than they actually are.
That said, they do make some amazing tapes, especially in acoustic or other low spl situations. I ran them for 2 years and did make some great tapes, but not at really loud rock shows, there are better mics for rock. I do think that the omni cap is somewhat an exception to this rule, it sounds amazing in most outdoor situations I've heard it in. The hypers lack presence and offer the worst characteristics of all the MBHO caps. This is all just my opinion, YMMV.