The attached footage was uploaded to XboxDVR.com and summarily removed, but not before hitting NeoGAF, MEGA cloud storage and then the rest of the internet.
The video was created using Upload Studio, as such it's only running at 720p and 30 fps above. YouTuber EKM Krauser reports that the game runs at 60 fps and enjoys improvements to lighting, blood effects, textures and new smoke effects. I'd hazard a guess that the full game runs above 720p as well.
Examining the footage, the gore effects certainly seem exaggerated over the original, and the lighting bouncing off the damp cobble stones was certainly not present in the original 360 version. The texturing also seems improved even when compared to the PC version on max settings.
The footage certainly seems genuine at a glance. However, it seems like a massive oversight for Microsoft to allow any supposed Gears of War: Ultimate Edition beta to record Upload Studio clips. Developers have the ability to prevent Game DVR from running captures in pre-release games, so it seems odd that the option to block it was overlooked.
I think it's entirely likely that this footage is real, but as with anything online, take the leak with a pinch of salt until we get official word from Microsoft.