The funny thing about specialists is that they can easily embarrass themselves when outside their field.
Watching your average politician talk about technology is a pretty great example of people speaking with zero context. Some of them clearly aren't dumb but they can get in over their heads with ease.
I seriously doubt that, actually. Are they more musically gifted? Sure. Smarter overall? I'm not so sure. It would be an interesting study.
I imagine the ones who are both successful musicians and businesspeople (i.e., they run the business themselves mostly, in addition to being musicians) probably are very intelligent, because that requires a broad range of skills. The ones who are just good at playing an instrument or singing some lyrics that someone else wrote? Not so much.
the best sports players and best musicians I would also suppose were higher IQ on average than normal sports players (I suppose this term means professional athletes) and normal musicians (professional musicians)
In the musicians category I wouldn't be surprised if the best musicians were also smarter than the general population.
In short I think IQ tests are a lousy metric, sure, but it is not a completely useless measure - just nearly useless.
On edit: clarification, fixed wrong capitalization.