Agree with all of this. But would add.....Depends. I do.
Most hardcore weight weenies have a "show spec" version of the bike (all but unrideable) and a "race version" which usually adds a kilo in wheels, tyres and pedals.
Notable exceptions to this are Dangerholm, who has some serious issues. Serious issues that i wish i had, total and utter focus on detail and a shit load of spare time/cycling related job, so you end up with 20lb freeride bikes and full on DH bikes at under 30lbs.
The other notable exception is SebK who is so fast on his lightweight XC bike that no one has ever seen him ride it.
OP, you're unlikely to find many light, carbon XC frames with canti bosses.
A move to discs will get you something like a scale, which at it's lightest was under 900 grams. (i built one on a ~£2k budget with discs, front suspension and 2x9 that was about 18lbs with some fast rolling tyres on it. Could easily have shed another 3lbs with another 1500 quid!.)
You are unlikely to find a superlight carbon xc bike with cantilever bosses, a straight steerer (this is a pre 97 thread afterall) and 26 inch wheels.
Should the thread have been. Let's build the lightest 26er retro ride it might be different. But you aren't going to be able to build anything in the dangerholm mould without serious skills or $$$$$$
As I said even a stock mondraker podium in medium is 775g