It won't be that much more powerful than the current leaks if it uses the 360 SOC for extra processing. The 360 is PowerPC based and the 720 CPU is x86-64. It would be a programming nightmare. The GPU wouldn't add much either. It certainly would not make up for the >600 gflop difference in GPU performance between the 720 and PS4. It would make up 150 gflops at best.
Runs Win 8? Lol yay. More APIs for the system to chug through while also trying to run games.
Apparently only an hour long event. So don't expect them to show as much as Sony. Though they almost certainly will show the design of the system.