Video compression ratios and transcoding

Not sure you'll find what you're looking for.  The only https://docs.microsoft.com/en-us/stream/office-365-video-feature-breakdown I have seen is related to Office 365 Video, the more established video service that's being replaced by Stream later in the year.

You can get a deep dive into the mechanics of Microsoft Stream in the presentation here - https://stream.microsoft.com/en-gb/blog/learn-how-microsoft-stream-was-built-using-azure/ and you could always check out https://azure.microsoft.com/services/media-services/ accordingly, which provides some of these related services that are mentioned.

Since every video is different, which you will know better than me, how it might be rendered and under what conditions it is played could vary, personally, I'd test Stream with sample content and do an analysis, as part of an evaluation. 

You can always sign up for a dev tenant - https://developer.microsoft.com/en-us/office/dev-program to help with this.  Also, you'll want to be aware of some of the current limitations like no public (anonymous) access to videos, API access etc., if you don't know about these already.