I was also hoping to hear from a Bose representative directly that can answer specifically if the rear surround speakers will be lowered or diminished in volume or features by activating dialogue mode.
Listen for yourself. What's important is what you perceive. I sometimes have to turn down the volume of the system when using dialogue mode. So, I'm just increasing the front center volume relative to all the other speakers. If you don't like how it sounds, don't use it.