Isn't enabling Turbo just saying "now the content will run through our servers and be compressed"? Amazon's Silk browser can switch between the 2 modes, too, as far as I know, although it uses the server compression by default.
The website is sent to Opera's servers to be compressed into opera binary markup language, not gzipped html. Probably presto-specific.
If Presto can convert pages to the binary markup language, why can't WebKit?
It can, of course, if you put in the work to make it do so. Presto already has had that work done. I'm not sure doing the work for Webkit would be worth the reward, especially considering they wouldn't be in control of iOS Webkit (controlled exclusively by Apple) including their changes. But that's just idle guesswork based on a subjective view of costs and benefits involved.