Not sure where you read this, but I doubt you would see much range gain.
The design of the TX is such that it optimises battery power, so will continue to transmit at something near its rated power even with fading batteries.
I assume you have a Phantom with the 5.8GHz radio? The P2 with the 2.4GHz does not require any sort of booster, with the TX range exceeding the return light time of the quad.