i dont think there is any real difinitive way to tell how much power a game is using of a system. the current dev kits might be around 80%-90% of the final power but the interview took place when they only had alpha kits which were roughly 25% of the power.
some might say that if gears of war pushes only 80% out of a quarter of the system's power, then that would mean that gears of war only pushing a very small amount of the systems total power. we all know, however, that game development doesnt work that way. talking about how much a game pushes a system is actually pretty useless. different dev teams will learn to push the system in different ways, using different methods. unless the learning curve is to the point of frustration i believe that a console game can always be tweaked and improved apon.
my guess is that gears of war will be a great showcase for the system, but still not come close to the systems total power. just look at the splinter cell series. the first one was thought to have really pushed the xbox hard, but take a look at chaos theory and you will see how much power the first in the series really didnt utilize. once the development community will have come to grips on how to manipulate the multi-core architecture of the system and then the real showcase of the system will come out, and that still seems like atleast a year away. after that the dev teams can then tweak and configure their engines even further to give us games that truly push the 360