Pretty sure that what I stated is correct
Anyways, he's still not going to notice much of a difference from 140w to 200w here, especially givin the room size...
You are correct about the need to double the power (assuming the increase in SPL follow the increase in electrical power output linearly) output to increase SPL by 3 dB. I hate to say it but you were incorrect

in saying that it was Ohm's law that states that mathematical relation.
You don't have to be an EE or believe one but to understand what Ohm's Law is about, you can simply search the web for details. Just be careful about the sources. As for the sound pressure level calculations, definition of decibel, use of logarithmic scale, etc., there are plenty of easy to understand info on the web as well but again, go for reliable sources.
I agree with you point regarding 140W vs 200W not making much difference from SPL stand point. As a matter of fact, going from 140W to 200W would gain (10*log(200/140)=1.55 dB, definitely perceptible but it is generally believed that to perceive twice as loud, the SPL has to increase approx. 10 dB.