Looking at the OP ...
This isn't much of a problem with the XBox 360 having "Peaked too early" as much as it is the reality of videogame consoles at this point in a generation. Videogame consoles are never the most powerful hardware on the market, but are very powerful when compared to the hardware most PC gamers have when the console first lauches; and this results in very impressive games being produced for the system in its launch year and the year following. After a console has been on the market for a full 2 or 3 years, experienced developers who are working on their second project for the system will be getting just about as much out of the system as is possible. After you have hit this peak, most of the performance gains from the system are actually trade-offs of some form.
Now, after a console has been on the market for 2 or 3 years developers who are heavily focused on the latest and greatest technology start looking towards what is possible on the PC (and what will be possible on next generation consoles).
In the past, systems like the Playstation had (what seemed like) massive improvements in hardware utilization later in their life but this had very little to do with an ability to get more performance out of hardware. A lot of this boost came from the fact that 3D was very new to most developers, and as better optimization techniques and algorithms were being developed by people like John Carmack they were being shared with the industry through new events like the Game Developers Conference.
People who expect drastic improvements from what the PS3 and XBox 360 are producing are mostly delusional.
