mhchem is nice in principle, but we can live without it. I'm not quite sure which renderer we are using at the moment, but I can assure you that it won't be trivial to get a new package installed.
I think switching to MathJax as the latex renderer is a good option. It's more or less the standard on the internet now (created by the American Maths Society; wikipedia, stackexchange etc. use it) As far as I can see there's not an SMF module for it, but works based on js script so I'm assuming it'd be possible to set it up. It looks like some LaTeX packages can be enabled http://docs.mathjax.org/en/latest/tex.html#mhchem, such as mhchem. I imagine it would break compatibility with past posts, but if disabling the current LaTeX renderer would leave posts looking like [tex]stuff[/tex] then setting MathJax to use those in addition to the $$ symbols it uses by default would keep those posts working.
It'll switch rendering to be client-side so there shouldn't be an issue of it overloading the AN server, and it'll be better than the LaTeX server we're using at the moment. Works on mobile phones too.
The issue is we don't want to keep playing with the SMF backend, for the problem that upgrading in the future would be problematic. MathJax can make life a little harder, since you'll either need to write a module around the MathJax API to plug into the SMF, or you'll have to hard-code it. Either requires a lot of effort, for not too much gain.
It'll be a different question if there is already a SMF module for MathJax though.