I doubt that "someone in the industry" is correct. The extra reserves just don't come into play at low volume. In fact, the opposite is more likely true. A lower powered amp will be operating in its comfort zone, and many audiophiles believe that sounds better than using too big an amp that is loafing along.
On the other hand, a well set up dual-mono setup may have certain other advantages, and if you occasionally play loud music - symphonies more or less require it, or you can't hear the quiet passages - that's another matter.
On the other hand, a well set up dual-mono setup may have certain other advantages, and if you occasionally play loud music - symphonies more or less require it, or you can't hear the quiet passages - that's another matter.