I have a 2003 Formula Fastech 382 with Mercury Racing HP525's and Mercury Bravo XZ drives. The starboard drive failed after about 155 hours and the port drive failed about 10 hours later. Am I the most unlucky man on the water, or are there known issues with the XZ drives?

Your "normal". The problem is the 11-12,000 lbs in front of those drives. If I remember correct, XZ's aren't much better than regular Bravos. I lost an XR on my 2002 382 and I tore the drives down every winter to inspect and changed the fluid every 25 hours.