Well, that's not really absurd or far for the truth. Music is selected for its effects on the consumer in all kinds of commercial establishments, and besides trial-and-error (this kind of music works best for people to drink more in our bar etc) there are lots of commercial research into it.

In the context of the story and Shereshevsky's personality, it is. To him, a person with extremely strong synesthesia, musical sounds literally have flavors, flavors that are as strong as actual real food to a normal person.

He isn't saying, "Restaurants play certain music to push customers to feel a certain way," he's literally saying that the flavors of music can match and enhance the flavors of food.

It could still be argued that the difference is in degree rather than in kind- that the 'extra bandwidth connecting his senses' allowed him to clearly articulate as sensation a phenomenon much more subconscious in the typical patron's mind.

No, not really. If you read the book, you will see that his mind was very idiosyncratic and the associations he made were highly unique and imagination-based, and not some sort of heightened sensory input.