A compressor will increase average loudness and thus power, but readability is reduced as it is increased and beyond a point very much. Those with hearing impairments may find readability reduced more. In practice 3W to 4.5W isn't much increase in range, so compressor may not help much compared to so simply adding a 20W PA, which on average with uncompressed SSB might look like 10W.
A peak reading power meter gives a better idea of power and a DX signal report of readability. Almost all power meters will give an average, not even RMS, with speech.
Mike EI9FEB