The developer kits in which those demos were made were under-clocked.
I doubt its from a developer system more likely a prototype box using a similar gpu and cpu combination. Remember the Wii U is a small console and will have to have very active powerful cooling if the gpu and cpu is too powerful. Also Nintendo cost down their products as much as possible. The original xbox had a much more powerful prototype than the final model for example. The prototype had a 300mhz gpu, more memory and faster cpu but the final xbox was 233mhz gpu etc.
Most of the demos at E3 were from ps3/360 games for the wii u and Nintendo were happy to use graphics from those consoles. They could easily have used PC graphics to demonstrate the Wii U graphics if it was much more powerful. One of the games on show at E3 was already in a playable state on Wii U hardware from a developer that had an early developement system but it was meant to be quite poor visually or at least inferior to current ps3/360 graphics.
I think expectations need to be realistic. Nintendo do not loss lead on hardware and if the specification is too good it would simply be too expensive for Nintendo to make a profit on especially if it doesn't launch well and has to drop in price. I would say compared to existing models (360/PS3) the Wii U will be somewhere between 80% and 150% of their power.
It wouldn't surprise me if the powerpc chip was something like a dual core 2.4ghz chip and the main gpu was running at 300mhz and a fairly limited model. However if the console has lets say 1GB of main memory and 32 megabytes of ultrafast video memory it will still have some big advantages over 360/PS3. If they combine the cpu, gpu and 32megabytes of video memory all into one large i.c maybe at 28Nm then it could still outclasse the 360 and PS3 in many ways and perform as well. PS3 performance is limited by memory bandwidth and lack of memory, 360 is really limited again by memory and also by its small capacity dvd drive.
Anyway I'm sure the specification will get leaked when the development kits are delivered.
As always it will probably be a mixture of good and bad.
