I would argue that differences in tire construction and materials would make MORE of a difference when the tires are such a big part of the weight of the bike. Reducing rotating weight by 20% means a lot more with a 1500g tire than with a 250g tire.
Also, suppleness makes a bigger difference the lower the tire pressure. The whole point of fat tires off road is that they conform to the terrain. The more supple the tire, the easier it conforms to the terrain and the less energy wasted in doing so.
IMO, the money spent reducing total tire weight by 1lb with better materials and construction is way better spent than reducing frame weight by that much.