I want to say I read somewhere that, particularly with heavier lenses (such as the 70-200 F/2.8), having two batteries in the portrait grip would provide extra juice to the AF motor and help it achieve focus faster. Can anyone confirm if this is accurate?
This is completely dependent on the make/model of the DSLR. Some ONLY use the extra battery power as reserve, while others will register the availability of the extra power and drive the AF motor more vigorously.
But this is brand/make/model specific.