Linkwitz uses the Millenium in his Orion, so it can't be too shabby.
Yeah, but SL used a 1440Hz crossover to the tweeter, and having heard the Millenium in the Orion I wasn't especially impressed, compared to the B&W Diamond tweeter or the Revel Be tweeter. I did hear them in a Joseph design once, and I thought it sounded better than on the Orion, but I wasn't listening with familiar music, so I can't be sure.
I agree about the OmniMic challenges, though they appear to have made things a lot better with the follow-on version (OmniMic V2). Having had both, the V2 seems to be more accurate. A friend with an XTZ compared it to my OmniMic V2 on my system, and the results were satisfyingly close. Not perfect, but close enough for my use of it.