600 carbs is a shitload, that's for sure. I recall Layne being up to 400+ and a natural xfit buddy of mine who was INCREDIBLY conditioned and strong, averaged 400-500. I'm only in the 200 neighborhood but I've been sorta fixing my nutrition for the past 6 months. so far I've been able to add about 1k calories per day without gaining weight so while I don't think I'd ever hit 600 without putting on some chunk, 300+ might be achievable.
hard to choke down 600g of cho though when you still have some protein and fats to find lol