Browsers are a hard thing to compare because you're really not comparing application frameworks, you're comparing engines. In the case of the Jolla browser, you're comparing a small Qt-based UI shim plus the Gecko rendering engine doing all of the work — a massively complicated, large codebase, against whatever else you're using.

Having used WebEngine in non-Jolla projects, I'm happy with it. It's a very nice tool, and I'd be very interested to see it in an experimental browser.

I'm not completely sure it would be able to replace the built in browser overnight, though: I don't have numbers, but WebEngine was quite resource-hungry.

You would be able to (independently from Sailfish) build your own copy of a newer Qt (including WebEngine) and use that to build your own application. I'm fairly sure that Silica's UI components wouldn't work though, as last time I looked, they had dependencies on internal APIs inside Qt (which have no backwards-compatibility promise, meaning they break from one release to another).