A Sub would offload the bass when bonded to an Arc. You can’t bond a Five to an Arc, you could group them, but that wouldn’t change the sound profile of the Arc. 

Sonos doesn’t advertise anything they sell as a ‘studio monitor’. Perhaps as there is no real definition of such. Much of what you’re asking about are personal opinions, so it’s hard to say if bonding a Sub will do what you want, but Sonos does have a return period, you could purchase one, and see if it meets your expectations or not. 

From my perspective, a sound bar will never be close to a studio monitor, they’re built for wildly different uses. A sound bar contains three channels, and is designed for connection to a TV, so it’s relatively smaller by its very nature, whereas a studio monitor is designed for a single channel, and is not restricted by size, so the physics would be substantially different. But I do own a couple of Arcs, both are bonded to Subs, and they’re extremely effective sound bars that work amazingly well with the TVs that they’re connected to. If I was looking for something closer to a studio monitor, I’d be much more likely to look at a Sonos Five, with a bonded Sub….although the Five does fairly impressively without a Sub.