The currently 'most extreme' XC bike that I know that is being ridden in XCO is the Arc8 Evolve FS, which has a HA of 65°. I have test-ridden it myself, and the very slack head angle doesn't help handling in slow, tight corners. The faster you go, the better it works. I liked it, but I'm not a purebred xc rider. My current bike (Mondraker Raze) which is 150/135 has 65,5° and would be classified as Trail/All Mountain, to put this in perspective.
These new geometries shouldn't be combined with stems longer than 50mm, and the fork should have a lower offset (normally 42 or 44).
Seat angles are a mess of its own, since companies have various ways to measure seat angle (the actual tube angle, the real angle at stack height..) and while their diagrams sometimes let you work out which one they mean, I would always take seat angles with a pinch of salt. For general reference: 75-76° is quite normal today, I have seen extremes of 78°+. My current bike has an 76,5° SA (72° real tube angle) and it works good for me. A bigger rider with more seatpost out might find himself further to the rear than he'd like, as the seat angle slackens the higher you go. Also, it depends on your crank length (shorter -> further back), obviously on your anatomy and your preferences. The old "knee above pedal axle" bike fitting method also has to be dropped with such seat angles.
Having started mtb on an early 29er XC bike with 69° HA, the ride characteristics of the current bikes are vastly superior. Much more stable, much less prone to OTBs. Obviously, dropper posts also play a major part in this - no more hanging off the back. What would have been an downhill/enduro bike geometry 10 years back is downcountry now. So you have a much better bike on the downs (mind you, with less suspension and durability than an enduro, so you shouldn't ride it quite like one) without really giving up anything for it on the way up.
