I found that interesting too. I'm not familiar with office 365, but isn't it web based? So why does it matter what browser or OS you use?
Keep in mind, XMLHttpRequest was introduced by MS in IE, and then was adopted by other browsers. It's likely that some Google products use Chrome specific extensions if they detect Chrome. If you can provide a better experience, it's often worth doing so.
Unfortunately there may occasionally be problems where falling back to standard features may be buggy, or development has continued assuming the existence of proprietary features that has led to designs where the fallback to standard features isn't just a little slower, but massively slower, because it might be a less checked test case.
In other words, I think this is far more likely to be an oversight because of a poor testing regime than a purposeful action.
Regardless of your browser, it'll heat your system up so you can fry an egg or broil a steak.
Something to do with WebGL being used during background transitions would drop the performance of my graphics card into the dirt. Games running at the same time would incur a frame drop of 90% for the 5 seconds Hangouts is merely changing a PICTURE..
Ironically that's being caused by WebRTC not having compatible enough implementations across all major browsers (and as an aside being disproportionally difficult to debug). Otherwise they would've just dropped the plugin and be done with it much earlier. Though I'm surprised they didn't act earlier.
There may be some cloud integration to OneDrive or something similar, but I haven't used that. It is nothing like Google Docs.