I've done super light hardtail before when i was 'fresh' to the MTB scene. I bought a Scott scale 20 off ebay with about 30 miles on the clock without really knowing what it was about. It was very quick but also felt very nervous/twitchy with my considerable stamp perched on it. Although i am a fan of carbon, having owned the Scale, i think i would shy away from it this time.
I have heard that Ti is more forgiving and 'springy' and would love a 456. But…
How much does the FS really make a difference towards the end of a grueling event? And is it worth the extra weight? I'm thinking it might. And then could also be used in other guises (i'e xc) if required to do so in the future. It would be a more versatile machine is what i'm trying to say.