Rage3D ran some benchmarks with Assassin's Creed and discovered the impact that DX10.1 has on this game is much greater than expected:
The impact that DX10.1 has is quite frankly greater than expected. A 20% improvement in AA performance is very impressive. GPU refreshes have been built around lesser gains. Based on the game's behavior it's possible that Ubi are doing their own Custom Resolve in order to get HDR correct AA, and are also taking advantage of the full access to MSAA buffers DX10.1 brings in order to get both better quality and better performance, but without an official statement we'd rather refrain from speculating further.