When are you seeing the clipping? On the camera, in Aperture? or after opening the photo in Photoshop as a TIFF, PSD or JPEG?
If the latter is the case which color space are you assigning for post-raw processing?
When you say "extreme highlights" could those be specular highlights - light sources in the photo or reflections of light sources in the photo?
Is is possible you've got some areas that are beyond the range of your histogram?
In Aperture, I'm using Adobe 1998 colour space. Not specular, mostly clouds or white buildings in full Sun.
I've not done any formal testing on this, I usually see it in white clouds. It's more a curiosity than serious problem.
I've never had it with other cameras, I'm usually deciding where exactly i'm prepared to clip the highlight as the histogram extends further.
Not the X, I often get a histogram that fits , if I just used the histogram I would often be bringing in the shadows and highlights points to the ends of the h/gram.
As I say more a curiosity than a problem as I go by eye anyway, the h/gram is just another guide.