The two lower-voltage batteries in a grip are used alternately, not simultaneously, so no boost.
I didn't mean to imply that with a grip it can increase the AF speed. I was referring to the fact that as a battery is almost completely drained it can provide a slightly lower voltage. So a battery grip, since it provides more total energy stored, can keep the rated voltage longer.
The power and voltage of a Li-on battery does not drop off like a regular battery. Circuitry internal to the battery monitors it, and throws a electronic switch when it senses that it is nearly discharged. Any drop in voltage is very small.
This is noticibly apparent in my DeWalt Drill Driver which uses a Li-on battery. It drives screws, even difficult ones with full strength, and suddenly shuts off while driving a screw, it does not fade away like nimh ot nicad batteries.
There is a good reason for this, a Li-on battery cannot stand being fully discharged, if that happens, it cannot be recharged without some special equipment. So the battery may shut down when it gets to 10% or so of capacity.