Well anyways, I'm still holding out hope that the XP8's will be quiet enough on the street regardless. If not then I'll look at my street options for rotor-compat. It seems like the reviews on XP8 noise vary a lot, and I'm starting to think this is mostly to do with user/install variations. Seems like probably the key is to properly lube them with disc brake lube of sufficiently high temp rating, and to bed them in fully and properly, both of which I intend to get done right.
As far as lube goes, going to give this a try. The temp range on it is out of this world, way beyond temps that would destroy the rest of my brake hardware:
http://www.permatex.com/products/Aut..._Lubricant.htm