What is behavior of this virtual ground circuit when sourcing/sinking current?
It's amazing what passes for circuit design in some of the DIY forums.

Your link goes to this schematic:


The center point created is limited by the charge on the 220 uF capacitors, so particularly at high amplitude/low frequencies (as the amp draws longer current and perhaps asymmetrical pulses) the center point will wander. 

This is just the same as if the amplifier output had been capacitor coupled which negates what was probably thought of an advantage (DC coupled output).

To fix up this amplifier, I'd simply use a 2 pole supply switch and ground the center point of the batteries ...Voila ....dual supplies and proper DC coupling.