It appears, the U400 is rather special in that regard. I tried searching for it and came up with this thread where similar performance problems are discussed. Looks like the driver from Lenovo is somehow messed up and you indeed cannot select a GPU from the BIOS. One person could fix it by a factory reset and thus restoring the original drivers that came with the notebook that - according to him - work with the different performance modes.
I'm afraid there's not much you can do, as the Omega-Drivers might indeed mess with your configuration since it seems Lenovo made a rather strange dual GPU configuration that is prone to errors...