Seriously, a working USB to SD-Adapter could theoratically work miracles. Everybody's talking about transfer-speed, though to me it doesn't seem to be the real issue. I read in another thread, that transfer speed on the SD-Port is limited to about 25 MB/s. This is why super fast SD-Cards don't load much faster than standard ones. I also read that the WiiU's drive only reads with about 60 MB/s or so. If that's correct, all the games should "only" take about twice the time loading. But as we all know, many games take much more time, especially those who consist of lots of small files. So the real problem is the internal data allocation of the SD-Card, the step BEFORE the actual data-transfer. A USB-HDD would provide a much faster internal file access, even though the following transfer is limited by the speed of the SD-Port.
The benefit could be that games, which have 10 times longer loading time could speed up to only needing twice the time to load compared to the disc.