Memory bandwidth isn't as important on consoles as compared to say... The PC.
1080P and lower resolutions, low levels of AA and the streaming of data from an Optical Disc isn't that demanding.
I doubt that the lower bandwidth would have much of an effect on fillrate either until later on in the consoles life.
With that said, the WiiU like most computer devices works on a series of caches which increases in speed the higher up in the cache hierarchy you go, reason for this is to hide the higher latencies and bandwidth of the slower caches.
At the bottom you have the hard drive/optical disc, then the storage mediums cache, then system ram then to the processor/GPU's cache and so on. (Many more in between, but you get the jist)
The end result is, Ram is just another cache, depending on the other components of the console and how developers utilize it, it's really not important on how fast it is.
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