I'm the author of toneAC and a Teensy 2.0/3.0 owner.
I've found that toneAC will make piezo or a standard 8 Ohm speaker much louder with a resistor. It's not quite twice as loud, but close. The entire purpose of creating toneAC was to generate higher sound pressure levels (higher volume). It's not an AC signal, it's just two pins generating square waves that are 180 degrees out of phase from each other. So in a way, instead of driving a speaker at 5 volts, you're driving it at 10 volts. This generates a higher volume of sound. 6dB would be twice as loud, and twice the voltage is about 5dB louder in my tests with toneAC.
Basically, with just a standard DC square wave you're only ever driving the speaker/piezo membrane in one direction. With toneAC, you're driving it in both directions so the membrane moves twice as far. This generates a higher volume level. The other advantage is that it's better for the piezo membrane as driving it only in one direction will cause it to deform over time. Sending the piezo an alternating polarity keeps the stress equal in both directions which won't cause it to deform.
In any case. This is all true if running toneAC on the ATmega. While the Teensy 3.0 modifications will play tones, it's not louder than any other method of sound output as it only drives one pin.
Tim Eckel