Improve RF range with a boost converter?
The output power will be maximum at maximum input voltage. It just mentions a typical and maximum: 14dBm typical, which is 25mW, and 32mW maximum.  

But it also says maximum supply current is 8mA. At a low supply voltage like 1.5V that would mean it consumes 12mW. In that case it can't possible transmit 25mW, so at lower voltages the power will be indeed limited.