This is completely untrue. The Apple A5 is based on the Cortex A8. It is about as fast as the Tegra 2.
The moment they do that the Android project is finished.
With full access to the source hardware manufacturers will have a field day trying to create custom versions for their phones, and within a year Android will cease to exist, being replaced by Android Sense, Android TouchWiz, Android MotoBlur, etc... Since they will be so fundamentally different that nobody will be able to develop apps for all of them.
Imagine if Gnome and KDE were proprietary, so that if you wanted to run a GTK app you would need to install Gnome, and Gnome would only be available on Fedora. If you wanted to run a Qt app you would need KDE, which would only be available on openSUSE.
That's what will happen if Android goes full open source.
Also, Apple will probably sue Google for infringing on 500 bogus patents once they dig evidence out of the source and twist it around until it says what they want it to say.
If anything Android is too open source.
edit: also worth noting is that none of the more open OS's have been injured by being more open, but instead have tended to be more unified and avoiding fragmentation.