Here is my measurements list:
Notubes Crow: 439 / 478
Bontrager XR1: 484 / 489
Kenda Karma 1.9": 521 / 498
Notubes Raven: 513 / 517
Schwalbe Racing Ralph 2.25": 563 / 547
Continental Race King 2.2": 647 / 659
Bontrager ACX 2.2" TLR: 656
From 550gr upwards, I have a hard time to consider tires to be race-worthy. That's a very personal limit, very much influenced by the terrain I race on: Mostly forest conditions with little rocks thrown in. I find, there is very little choice under 550gr. It seems some tire manufacturers just do not push to low weight in the 29" department as they do with 26" tires. There are masses of 29" tires above 600gr because of that and I think they do not need to be.
The ACX kind of proves that. Full knob, pretty wide and TLR beads at a pretty good weight. Conti achieves the same weight with their Race Kings (and my pair is light!), but with only a semi knob tread. If Conti would use their "supersonic" casing, the Race King would be on par with the Racing Ralph, possibly even lighter. Now THAT would make a racing tire. We're only at the beginning of 29er specific development, I guess.
Now if we just could persuade the Germans to go and race 29ers...