In emacs I represent everything in Lisp and I use separate color schemes for SXML documents and code. This effectively allows me to distinguish between these different classes of data. Furthermore, emacs has different sets of functionality for different types of data, so I feel that I have all the features you mentioned already.
It's unsurprising you mention sxml given that XML and s-exprs have a trivial homomorphism. With that said, I'm also not a big fan for representing relational data in XML (or sexprs).
Having a uniform representation for data isn't a big enough win to trump having data represented in a way that is more natural for me to think about.
With that said, if your brain thinks in XML (or sexprs) maybe Lisp will always work best for you.
