Your thinking is too linear. Radio doesn't work in linear calculations.
Transmitter power is only one of the dozens of factors that go into RF propagation calculations. Other factors include distance, altitude, fresnel zone interference, free-space loss, cable and connector losses, antenna efficiency and receiver sensitivity.
Since we're digital, the signal is either there or not. There is no in-between. Calculating radio range is serious equations, explanations, tutorials and other brain-hurting material from people who really know their stuff. (I spent a whole semester on this subject). The rule of thumb for all digital wireless systems is have at least 30% more power than you need with a minimum of 5 - 10 dB. This is called the System Operating Margin (SOM).
The difference in the SOM from 520mw to 746mw is negligible. It is from 12dB to 13.57dB according to calculations on Radiolabs.com and assuming everything else is constant. The difference in range would be measured in inches.