My experience is that the "boutique" gearmakers are generally honest about their gear specs. In no particular order, ULA, Six Moon Designs, Henry Shires, GoLite, Western Mountaineering and MontBell (a Japanese co. that's relatively small here but big in Asia)...
As for the larger gearmakers, I have found their gear specs to be off (and it's much more often heavier/colder rather than lighter/warmer than stated)! Marmot, Eureka, Osprey, the list goes on and on. Slumberjack is particularly egregious in bag warmth rating. And in the travel world, Eagle Creek packs can be significantly heavier than advertised. One "big guy" exception is REI. Their specs are generally accurate.